Materialized views for documents

[loraderon]

When using Marten as a document database, there seems to be some missing funtionality with joins/include:

• Support Linq GroupJoin Operator #1599
• Projection for included documents #3095
• Combining Include, Where & Select ignores Where #3096
• What would it take for you to adopt Marten? #1701 (comment)
• Using Include, it's not possible to filter on the included document properties #3120

One way to support the workflow would be to add support for querying materialized views using Marten.

There would also be a need for some sort of view builder. The view builder could be a custom api instead of relying on Linq.