feat(oauth2): persist refresh token between Toolbox restarts

[fioan89]

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.

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