feat: implement OAuth2 client credentials flow to access API

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To demonstrate usage of Ory Hydra, let's implement the following use case:

• A user can create clients (used in applications) to act on their behalf
• The client will be bound to the user with the owner property during the client creation
• The client will be replicated into the application's database
• When requesting an OAuth2 token extra information will be appended to the access_token (using the Ory Hydra webhook)
• When sending requests authenticated with an access token (ory_at_xxx), a guard should validate the access token with the introspection method from OryOauth2Service (Consider using or-guard with OryAuthenticationGuard and OryOAuth2AuthenticationGuard)
• Check authorization using the permissions of the client's owner (with Keto) the scopes contained in the access token which is a bit of an OAuth2 spec violation but we will survive it!