Pack ContentItem with fields

[nkolev92]

Bug

Fixes: https://github.com/NuGet/Home/issues/12728

Description

This saves us about 40MB.

Before:

Name                                                                                                                                                                                                                     	Inc %	          Inc
 nuget.packaging!NuGet.ContentModel.Infrastructure.PatternExpression+TokenSegment.TryMatch(class NuGet.ContentModel.ContentItem&,class System.String,class System.Collections.Generic.IReadOnlyDictionary`2,int32,int32&)	  0.9	   76,216,336
+ mscorlib.ni!System.Collections.Generic.Dictionary`2[System.__Canon,System.__Canon].Insert(System.__Canon, System.__Canon, Boolean)                                                                                     	  0.4	   34,115,800
+ nuget.packaging!NuGet.ContentModel.ContentItem.get_Properties()                                                                                                                                                        	  0.2	   18,992,256
+ nuget.packaging!ContentPropertyDefinition.TryLookup                                                                                                                                                                    	  0.1	   12,430,096
+ clr!JIT_New                                                                                                                                                                                                            	  0.1	   10,678,184

After:

Name                                                                                                                                                                                                                     	Inc %	          Inc
 nuget.packaging!NuGet.ContentModel.Infrastructure.PatternExpression+TokenSegment.TryMatch(class NuGet.ContentModel.ContentItem&,class System.String,class System.Collections.Generic.IReadOnlyDictionary`2,int32,int32&)	  0.5	   36,278,392
+ clr!JIT_New                                                                                                                                                                                                            	  0.3	   25,499,680
|+ clr!WKS::GCHeap::Alloc                                                                                                                                                                                                	  0.3	   25,499,680
| + clr!WKS::gc_heap::try_allocate_more_space                                                                                                                                                                            	  0.3	   25,499,680
|  + clr!GCToCLREventSink::FireGCAllocationTick_V3                                                                                                                                                                       	  0.3	   25,499,680
|   + clr!CoTemplate_qqhxpzqp                                                                                                                                                                                            	  0.3	   25,499,680
|    + clr!EtwCallout                                                                                                                                                                                                    	  0.3	   25,499,680
|     + clr!ETW::SamplingLog::SendStackTrace                                                                                                                                                                             	  0.3	   25,499,680
|      + Type NuGet.ContentModel.ContentItem                                                                                                                                                                             	  0.3	   25,499,680
+ nuget.packaging!ContentPropertyDefinition.TryLookup                                                                                                                                                                    	  0.1	   10,778,712

Some micro benchmarks:

Before:

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
NJ	40.24 us	0.353 us	0.331 us	3.2959	0.1221	56.5 KB
MicrosoftBuildRuntime	62.62 us	0.822 us	0.686 us	8.1787	0.6104	138.23 KB
AllPackages	152,979.21 us	804.331 us	713.019 us	14250.0000	2000.0000	240733.08 KB

After:

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
NJ	34.69 us	0.257 us	0.228 us	2.3193	0.0610	39.92 KB
MicrosoftBuildRuntime	58.17 us	0.680 us	0.636 us	7.3853	0.4883	125.22 KB
AllPackages	133,135.56 us	669.977 us	626.697 us	11000.0000	1000.0000	185389.49 KB

PR Checklist

• [ ] Meaningful title, helpful description and a linked NuGet/Home issue
• [ ] Added tests
• [ ] Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.