Refactor EntityMetadata to improve state consumption

[jviau]

This PR changes to following:

1. includeState is true by default for all get entity methods (including get all)
2. Added EntityMetadata<TState>, which has a strongly typed deserialized TState State property.
3. Extracted un-typed state into SerializedData, which has a ReadAs<T>() method.
4. EntityMetadata now implements EntityMetadata<SerializedData>
5. EntityMetadata<TState> now has a bool IncludesState which indicates if state was retrieved or not.
6. EntityMetadata<TState>.State now throws if IncludeState==false
7. Added GetEntityAsync<T> and GetAllEntitiesAsync<T> overloads to get EntityMetadata<T>

[sebastianburckhardt]

some suggested changes to the comments, and one question about a possible simplification.
Otherwise looks good to me.

[sebastianburckhardt]

we could perhaps implement this more simply as

public bool IncludesState => this.state is not null;

[jviau]

I don't have that to handle default value types. Basically, this has to be a pseudo Optional. Need to differentiate between state was fetched and it was a default value type, or state was not fetched at all (in which case the value is still default).