ExecuteUpdate: allow value tuples in setters for expressing row values

[roji]

context.Blogs.ExecuteUpdate(s => s
    .SetProperty(b => ValueTuple.Create(b.Title, b.Content), b => /* some subquery returning two values */);

This is important for performance, since the subquery runs only once and returns two values; otherwise the two properties need to be set separately, causing the subquery to be evaluated twice (that's the current workaround).

IIRC this does not work on SQL Server which doesn't support row values very well, but is supported on almost all other databases, including SQLite.

Possibly depends on:

• Allow users to express SQL row value comparison syntax #26822

[roji]

Note: we are not sure this is strictly necessary perf-wise; it should be possible to project before ExecuteUpdate, and then refer to the results of a subquery from that projection multiple times in the ExecuteUpdate setters. According to @smitpatel this may currently be blocked by some pending selector work, but that's something we'd want to do regardless.

So investigate whether this is actually needed.