Remove BsonClassMap.AutoMap from Mapping Registration Process

[Turnerj]

As MongoFramework implements its own mapping system, its time to replace the AutoMap call using the MongoDB convention pack.

The DefaultConventionPack uses the following:
https://github.com/mongodb/mongo-csharp-driver/blob/5d89fe5d928f8b4ef5476114dea3f9531b8dd94a/src/MongoDB.Bson/Serialization/Conventions/DefaultConventionPack.cs#L37-L47

There is also an AttributeConventionPack however seems mostly superfluous:
https://github.com/mongodb/mongo-csharp-driver/blob/5d89fe5d928f8b4ef5476114dea3f9531b8dd94a/src/MongoDB.Bson/Serialization/Conventions/AttributeConventionPack.cs

Of the default conventions, here is the state of things:

• LookupIdGeneratorConvention : Redundant due to EntityIdProcessor
• StringObjectIdIdGeneratorConvention : Redundant due to EntityIdProcessor
• NamedParameterCreatorMapConvention : Not redundant but not used either (plus likely won't be supported in the future)
• ImmutableTypeClassMapConvention : Not supported by MongoFramework
• IgnoreExtraElementsConvention : Redundant due to ExtraElementsProcessor
• NamedExtraElementsMemberConvention : Partially covered by MongoFramework (we use attributes, they use property name)
• NamedIdMemberConvention : Partially covered by MongoFramework (we use KeyAttribute in EntityIdProcessor, they use property name)
• ReadWriteMemberFinderConvention : Not implemented by MongoFramework

[Turnerj]

Of that list, here are my thoughts for what is left outstanding:

• NamedExtraElementsMemberConvention: This isn't (or maybe, "shouldn't") be a common scenario. You can just use the attribute if you need extra elements.
• NamedIdMemberConvention: This seems like a fair addition to have. Probably won't support "_id" but "id", "Id" and "ID" seem fair game.
• ReadWriteMemberFinderConvention: This is an important, and relatively simple, one. If implemented, I could probably change any other part of MongoFramework from manually doing reflection itself. It does mean dropping "fields" support entirely from the model but that is OK for now.