Add an API to set descriptor set and binding of default uniform block

[Akaricchi]

This is for the relaxed Vulkan rules mode.

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[Akaricchi]

I have signed the CLA, see here: https://github.com/google/shaderc/pull/1437/checks?check_run_id=27778572097

I don't know why the bot's comment is still up.

I'll see about adding the tests and CLI tomorrow.