tcgc: expose way to get default api-version on methods

[jhendrixMSFT]

Clear and concise description of the problem

While methods contain an array of available api-versions, there's no easy way to determine which one is the default api-version. At present, we can enumerate the params, looking for isApiVersionParam and clientDefaultValue. Maybe method types can have a method getDefaultApiVersion() or something that returns the default? Or the index into apiVersions that's the default version.

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.

[tadelesh]

apiVersion in method should be all api versions that contain that method and the default should be the last one in the array.