Add decorator to allow user-defined name for additional properties field

### Clear and concise description of the problem

There is no way at present to specify the field name for additional properties. It would be useful to have a decorator that can specify the name (storage would make use of this).

### 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 file, test, or entry point is named. Start by locating how additional-properties decorators and storage handling are implemented, then define the expected behavior for a user-supplied field name and add coverage showing that the name is honored.

Written by the indexing model from the issue text.