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

[Akaricchi]

This is for the relaxed Vulkan rules mode.

[google-cla]

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[dneto0]

Thanks for the contribution.

Please sign the CLA.

Please add a command line option for glslc, and add tests for the C++ API and the glslc command line.

[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.