Add a higher-order vector.extractelement operation in MLIR

This CL is step 2/n towards building a simple, programmable and portable vector abstraction in MLIR that can go all the way down to generating assembly vector code via LLVM's opt and llc tools.

This CL adds the vector.extractelement operation to the MLIR vector dialect as well as the appropriate roundtrip test. Lowering to LLVM will occur in the following CL.

PiperOrigin-RevId: 262545089