Add further support for managing OAuth2 client apps configurations

[ngilles-aiven]

No description provided.

[m-novikov]

Looks goot, but has issue with naming though. We generally use singluar forms for commands e.g. user-list so

• secrets-list -> secret-list
• redirects-list -> redirect-list
  Also verbs create instead of add and delete instead of remove

[lornajane]

If you're adding commands to the client, could you also add them to the docs, at least as a reference even if we build the proper usage example later? https://developer.aiven.io/docs/tools/cli.html

changes addressed