Detect breaking changes in api layer of modular client

[wanlwanl]

Due to

1. root layer may keep backward compability when api layer has breaking changes
2. decide to set hierarchyClient to true by deault according to subpath exports for hierarchyClient true export internal send and deserialize function autorest.typescript#2699 (comment)

[wanlwanl]

Hi @qiaozha , @MaryGao @joheredi I created this issue for supporting hierarchyClient better, and set an appropreate priority, could you comment in the issue about:

1. in what cases we have breaking changes in api layer while no in root layer?
2. what's the number of RP get impact?
3. case 1: only detect breaking changes in api layer vs case 2: detect breaking changes in both root layer and api layer: what the spec review flow will be? These two cases block PR merges? When to allow case 1 in spec review flow?