[TCGC] Add a clientOption(name, value, scope) decorator

[lmazuel]

Clear and concise description of the problem

We want to be able to pass experimental flags fast to emitter, without having to reship TCGC. For instance:

@clientOption("enableFeatureFoo", true, "python")
model MyModel {}

Target can be anything, name is a string, value can be anything (emitter's job to cast), and scope as usual.

We should have a permanent linter rule applied to that decorator, that's it's required to suppress. This is intended for experiment, or quick escape hatch and shouldn't be used outside of that context, or needs to be justified in the suppression.

Options of that decorator are NOT expected to be documented publicly, for the reason given before.

I expect this to be exposed in the TCGC YAML as something like a list:

clientOption:
- enableFeatureFoo: true

And any emitter can check if "clientOption" for something they support is there or not.

Checklist

• Follow our Code of Conduct
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