[TCGC] Add additionalApiVersion decorator

[lmazuel]

for instance your TypeSpec:

// Don't try to define the old things
enum Versions {  v4, v5 }

Your client.tsp:

@additionalApiVersions
enum AdditionalApiVersions { v1, v2, v3 }

And TCGC will expose the API versions enum as: v1, v2, v3, v4, v5.

The concept behind it, is that sometimes we want SDK to generate accurate API version enum (mostly C# and Java), while not having to actually annotate the entire spec with @added because we won't use versionning information in the generation side.