[TCGC] Add the possibility to silence warnings systemwide.

### Clear and concise description of the problem

It would be super useful, if we will be able to silence warnings systemwide as it is done for linter:
parameters:
```YAML
"service-dir":
default: "project"
emit:
- "@azure-tools/typespec-autorest"
options:
<...>
linter:
disable:
"@azure-tools/typespec-azure-core/casing-style": "Not now"
```

Maybe we can add the "disable" section for the emitter?
```YAML
"service-dir":
default: "project"
emit:
- "@azure-tools/typespec-autorest"
disable:
- "@azure-tools/typespec-autorest/union-unsupported": "Not now"
options:
<...>
linter:
disable:
"@azure-tools/typespec-azure-core/casing-style": "Not now"
```

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/azure/typespec-azure/blob/main/CODE_OF_CONDUCT.md)
- [x] Check that this issue is about the Azure libraries for typespec. For feature request in the typespec language or core libraries file it in the [TypeSpec repo](https://github.com/Microsoft/TypeSpec/issues/new/choose)
- [x] Read the [docs](https://azure.github.io/typespec-azure/).
- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

No source files, tests, or entry points are named. Start by tracing how the YAML emit and linter sections are parsed and how emitter warnings are reported. Done means a documented systemwide disable configuration for emitter warning identifiers, with tests covering suppression and the reason text while preserving existing emitter options.

Written by the indexing model from the issue text.