Query: Add support for custom EntityQueryables

[smitpatel]

This makes look up for EntityQueryable in query tree bit more complex but it adds support for providers to add custom EntityQueryable to carry more metadata as needs to and allow rest of the pipeline to be transparent about it.
Current FromSql has be to converted during compilation phase since, we want to parameterize the arguments array in the methodCall.
If there is nothing to parameterize then providers can directly create custom Queryable.

Resolves #16326

[smitpatel]

It failed to capture one particular case. (See #13346)
i.e. if the defining has FromSql then we need to run visitor to convert FromSql when expanding navigations. Again the information about this visitor won't be available in core level.

Another thing is, parameters passed to FromSql needs to be funcletized. Else we could just create FromSqlEntityQueryable from start when the method is called. Doing the later would mean that funcletizer needs to know about relational specific entity queryable.

For now, we should just pattern match on method name in core for FromSql and process it accordingly in core. It is somewhat hacky but this is only 1 method and any solution involves a lot of changes to allow this to work.
Closing this.