Introduce LeftJoin and RightJoin into .NET

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This user story tracks all the tasks needed to introduce the new LeftJoin and RightJoin operators into .NET, across the stack. It has nothing to do with EF specifically (except for the EF-side support in #12793), and is only here for tracking purposes.

TODO

• Proposal: introduce left and right join clauses to C# query expression syntax csharplang#8892 (open, awaiting reviewing)
• Support the new .NET 10 LeftJoin/RightJoin operators #12793 (in EF)
• API docs PR for the new operators
• Conceptual docs update (here)
• Analyzer with codefix, to transform the legacy combined approach to the new operator/language construct

Complete

• Runtime API proposal
• Runtime implementation PR