Allow tuples to be used as join keys

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When performing Join, EF currently only supports scalars and anonymous types (see #25075). The problem with anonymous types is that they can't be shared: you cannot have a property/function somewhere that's used in multiple queries, and which returns a key selector Expression<Func<...>>, since anonymous types are unspeakable and cannot appear in the Func signature (see #25075 (comment)). #25075 suggests allowing arbitrary POCOs as join keys, but that raises complex questions around equality behavior etc. which makes this undesirable.

A good solution here seems to be to allow tuples as join keys: these have clear equality semantics and can be named in Func signatures.