feat(api server): support show api servers

the api server's register info is not available from nameserver, this make it not possible to query api server info from sdk or command line, like SQL `SHOW COMPONENTS`

enhance #1232

To support show api servers:

1. in clustered mode: those info can be stored directly in zk, E.g `$root_path/apiservers`
- nameserver hold a cache set of all api servers, so it is as simple as query a RPC method from nameserver
- remember delete node info after api server lost for awhile
2. standalone mode:
- name server only hold the cache set, it do not write api server info into zk

The issue involves modifying the nameserver to cache API server information and handle registration/deletion in ZooKeeper for clustered mode, and manage a cache in standalone mode. Start by examining the nameserver's component management code and the ZooKeeper interaction logic. Look for existing RPC methods and the `SHOW COMPONENTS` SQL handling. Testing will require understanding the distributed setup and the API server registration flow.

Written by the indexing model from the issue text.