Enable the OAuth2 credential injector HTTPFilter

*Description*:
Currently the Envoy Gateway only allows to use the Credential Injector for Basic Auth with Proxied Requests to upstream. But what if we need JWT Tokens? Sure, there is the generic_secret and you can use a Kubernetes Secret to insert a JWT for Bearer auth - BUT: what if the tokens need to be refreshed regularly? There actually is an oauth2 credential injector according to the envoy proxy docs, but one cannot use it yet on the gateway, as the Envoy API Extensions only allows for a secret ref, not the configuration we'd need.

[optional *Relevant Links*:]
* [Oauth2 Credential Injector](https://www.envoyproxy.io/docs/envoy/latest/configuration/http/http_filters/credential_injector_filter#oauth2-credential-injector-client-credential-grant)
* [Envoy Gateway API Extentions](https://gateway.envoyproxy.io/v1.5/api/extension_types/#injectedcredential)