Refactor property info handling, allow keys different from Id property

[nseguin42]

I've moved this PR to a draft until I have the chance to investigate the test failures. In hindsight, it might be worth just changing GetSingleKey<T>() in a minimal way.

Behavior changes:

• In GetSingleKey<T>(), only return the property named 'Id' as a fallback if there is no Key or ExplicitKey property. (fixes https://github.com/DapperLib/Dapper.Contrib/issues/140) Note: The new behavior allows manually specifying a key (or explicit key) that is different from the 'Id' property. Any usage that would previously have succeeded (not thrown an exception) should be unaffected.

Refactors/implementation details:

• Introduce PropertyInfoWrapper to hold all the relevant information about a type's properties. This object is only mutated within a thread-safe Lazy<T>, so we shouldn't have any issues even if multiple threads grab the same instance from a ConcurrentDictionary.

• Move logic about selecting a key into the PropertyInfoWrapper and cache the result of GetSingleKey<T>().

• Add PropertyNamedId to distinguish properties named 'Id' from properties explicitly annotated with a KeyAttribute.

• Add an optional parameter to KeyPropertiesCache to specify whether you want to include the Id property or not, so that the existing code didn't need to be modified as much.