[Codegen][Tuner] Add pass to link tuning specs

[kuhar]

This pass is meant for combining multiple tuning specs (e.g., a user-provided one and a default one).
We expect the input module to have nested sub-modules with named sequences marked with the
iree_codegen.tuning_spec_entrypoint unit attributes.

The pass collects all such tuning specs and introduce a new named sequence that includes all the other tuning spec entry points. The order of inclusion is the same as the in which these nested tuning specs appear in the IR.

Issue: #19214