More consistent handling of extension data, for the client_credentials request
and respective endpoint and flow.

Feature release:
- Added `ClientCredentialsAddon` trait to modify the addon state during a
  `ClientCredentialsRequest`.
- Added `client_credentials` attribute to `AddonList`.
- Implement `Extension::client_credentials` for `AddonList`.
- Implement `ClientCredentialsAddon` and `Clone` for `AddonList`.
- Implement `Extension` for `&mut AddonList`.