Ability to map a CLR method returning queryable to TVF

[smitpatel]

Split off from #4319

            public IQueryable<OrderByYear> GetCustomerOrderCountByYear(int customerId)
            {
                return CreateQuery(() => GetCustomerOrderCountByYear(customerId));
            }

Above when used in LINQ will insert query root for OrderByYear alongwith TVF as SQL translation.

It also bootstrappable. So you don't need to create query root.

Pending items

• Map an entityType to TVF.
  • Ignore entity types mapped to TVF in migrations (don't map them to a table by default)
  • Query will use TVF whenever this query root is used.
• Convention to register keyless entity type returned by function.
• Store mapped EntityType in the IDbFunction (Store mapped EntityType in the IDbFunction #20160)
• Refactor IDbFunction metadata
  • Additional metadata to map queryable function related task
  • Store all functions in a single annotation
  • Dual facets for queryable & scalar functions
  • IDbFunction without method info
  • (Essentially make IDbFunction a TVF in "relational model"
• Query translation
• Apply null semantics processing to QueryableFunctionExpression (Query: Apply null semantics processing to QueryableFunctionExpression #20180)
• IDbFunction.HasTranslation API needs improvement (Disallow IDbFunction.HasTranslation when mapping the function to a TVF #20163)
• Consider a way to represent query roots which are custom like FromSql/Queryble method call (Query: Introduce an Expression for query root #20146)

[roji]

Adding providers-beware for the tests

[smitpatel]

Don't leak DbContext in query cache for queryable functions.