Secure storage of sensitive keys via OS Keyring

For smooth login experiences we should persist in in the OS credential manager (Keychain on macOS, Windows Credential Manager, libsecret/keyring on Linux) the following:
- client id
- client secret
- refresh token
Coder server implements Dynamic Client Registration (RFC 7591), each installation registers itself and gets its own unique client_id and client_secret, that can be stored in the OS keyring.
- CLI should also use keyring to securely persist access token in the OS Keyring

Other notes:
- access token, discard on exit
- access token expiry, needed only for the duration of the session
- code verifier, delete after token exchange
- state, delete after redirect verification