feat!(CosmosStore): Azure.Cosmos 'V4' support / enable mocking Cosmos APIs

[bartelink]

Takes master (which uses Cosmos SDK v3 (Microsoft.Azure.Cosmos) + makes it use V4 (Azure.Cosmos)

The diffs are fairly large between current master and the base of this PR; main reason being that Equinox.CosmosStore has cherry-picked the usage of System.Text.Json, leaving only two interesting facilities:

1. using the stillborn Azure.Cosmos package
2. introducing some mocking support, which I don't believe anyone has a use for

• [x] port logic
• [x] port to System.Text.Json
• [x] Merge #202 (which pushes some code in here out to https://github.com/jet/FsCodec/pull/38)
• [x] Merge #203 (@ylibrach's Client Abstractions work)
• [x] Merge #206 (@ylibrach's Unfolds Compression flag)
• [x] #208 Address failwith "TODO" re eqx dump functionality
• [x] #208 Review Internal things like Gateway and other such policy tweaks with a view to aggressively removing and/or de-emphasizing things in V1/V2 that are not being actively used
• [x] #216 Rename to Equinox.CosmosStore (with CosmosStoreConnection/Context/Category)
• [x] https://github.com/jet/FsCodec/pull/48 provide path to map between FsCodec.SystemTextJson and FsCodec.NewtonsoftJson
• [x] release FsCodec.SystemTextJson beta so we don't end up having a beta release depending on an alpha of something
• [x] Remove CancellationToken parameters
• [ ] @ylibrach renaming Equinox.CosmosStore.Core.ContainerGateway to CosmosContainerAdapter and any other desired changes
• [ ] @bartelink to release next alpha and validate on a branch in dotnet-templates
• [ ] ~Investigate RU consumption discrepancies first observed in #144 - atm there can be a 10% RU consumption increase, which seems (NOTE: wild speculation) to correlate with a lack of anticipated perf improvements when dealing with large spans of small events~ [master now as correct RU consumption using V3] now has correct RU consumption(https://github.com/Azure/azure-cosmos-dotnet-v3/issues/616#issuecomment-629813295), so there's reason to believe that this will eventually be righted in the V4 SDK too
• [ ] 🤔 Consider holding off until V4 reinstates Checkpoint and ChangeFeed Batch Context information
• [ ] Port skipping cert validation for emulators (done for V3, not possible in V4 yet)

[CLAassistant]

All committers have signed the CLA.

[bartelink]

🎉 Marking this ready for review; @ylibrach's hard work has reached a point where it's ready for much broader review.

Looking for comments regarding any significant problems triggered by breaking changes in this - we'll release a set of .alpha NuGet packages to enable test driving shortly

[bartelink]

@ylibrach I can cut an alpha.4 any time (I'ver merged in usage of the non-alpha release of FsCodec.SystemTextJson: v 2.1.0), but ideally you'd slip in naming changes etc you want to include

[bartelink]

Status update: This is parked for now, pending resumption of releases of V4 codeline from MS) a188f69 Presently, major features that were developed here are being cherry-picked onto master (which, for now, uses the Microsoft.Azure.Cosmos aka V3 API).

It's unclear at present whether the first non preview/rc Equinox.CosmosStore release will ultimately use Azure.Cosmos or Microsoft.Azure.Cosmos; the intention was and remains to move straight to V4 rather than forcing a transition (and/or the need to consider conflicts where codebases already use Microsoft.Azure.Cosmos) on consumers if it can at all be avoided (the OP details the concerns in more depth)