Generate models required for specific operations in REST API

[srnagar]

Design for specifying operations/operation groups that need models. The generator should include only models that are needed by the specified operations in public package. The generator, however, may decide to generate all models in implementation package.

[weidongxu-microsoft]

One issue is how to specify this in CADL. (assume we discard autorest and its md configs)

[weidongxu-microsoft]

For now, we will use a @convenienceMethod decorator on CADL operation, or on interface (group of multiple operations).

Then related model will get usage: convenience-method populated to all model/enum. And template will only write these models.

[weidongxu-microsoft]

#1670