Add a region to linalg.generic

This CL extends the Linalg GenericOp with an alternative way of specifying the body of the computation based on a single block region. The "fun" attribute becomes optional.
Either a SymbolRef "fun" attribute or a single block region must be specified to describe the side-effect-free computation. Upon lowering to loops, the new region body is inlined in the innermost loop.

The parser, verifier and pretty printer are extended.
Appropriate roundtrip, negative and lowering to loop tests are added.

PiperOrigin-RevId: 261895568