Flexible navigation mapping

[AndriySvyryd]

Navigations don't always have to use the same mapping as the entity type they are pointing to. Also, they don't need to be mutable.

• Separate update and query mappings, including readonly navigations
• Map to functions (Query: Additional function mapping capabilities #4319)
• Map to a custom LINQ/SQL query (Ability to define custom joins (for readonly navigations) #33171)
• Allow to specify a filter on a navigation

[ajcvickers]

See also discussion on #21991

[ajcvickers]

See also discussion in #24153