Provide a generic dictionary of decorators in TCGC for better extensibility

[live1206]

Clear and concise description of the problem

Nowadays when we add a new decorator, TCGC needs to process it and add a new property to SDK model.

We would like to have a generic dictionary to store decorators for emitter to consume, so that we don't need to add a new property to SDK model for better extensibility.

decorators
{
  "decoratorName":  string,   // The name should include namespace
  "decoratorArguments": string[]
}

Issues to address:

• how to filter which decorators to include in the generic dictionary
• how to present the decorator arguments/values in the dictionary, such as JSON serialization, need to analyze the existing decorators

Checklist

• Follow our Code of Conduct
• 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
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.