support client authentication

Created by: wansir

Signed-off-by: hongming hongming@kubesphere.io

What type of PR is this?

/kind api-change

What this PR does / why we need it:

For security reasons, client authentication for confidential clients is required when posting an access token request. client_secret and client_id are configurable.

Which issue(s) this PR fixes:

Fixes #2256 (closed)

Does this PR introduced a user-facing change?

None