Add a custom hook/extension mechanism for user defined passes

We've had requests from some ISVs to enable a way to hook new passes for validation and optimization without having to fork the codebase or add patches to the main codebase. Having a way to do this would allow some ISVs to more frequently update their versions of spirv-tools, by not requiring patch modifications, and instead making it more "plug'n'play". This is not a small effort, but recording it here to track its viability, and to focus discussion.

The issue names no files, tests, or entry points. Start by reviewing how SPIR-V Tools currently represents validation and optimization passes, then assess the proposed extension mechanism with the ongoing discussion; done means a concrete, agreed design for user-defined passes and its viability.

Written by the indexing model from the issue text.