[mlir][spirv] Add a field for client API in target environment

SPIR-V can be directly consumed by APIs like Vulkan and OpenCL,
where we can use the capability list to diffferentiate. It can
also be used as a compilation target to transcompile to shading
languages like WGSL to target WebGPU. We have no way to tell
that with just the capability list, so we cannot perform certain
transformations only applicable to those targets thus far. So
this commit add a field in the target environment to indicate
the client API for such purposes.

Reviewed By: kuhar

Differential Revision: https://reviews.llvm.org/D138732