[Request] Allow oauth2_proxy configuration passthrough

### Describe the feature

https://github.com/cryostatio/cryostat/issues/629

If the Operator allows full configuration of the oauth2_proxy, then not only can users create Basic auth configurations for user auth, but could also configure the proxy to delegate to a proper external OAuth provider. This would be more on par with the OpenShift OAuth Proxy.

### Anything other information?

_No response_

Read the linked Cryostat issue #629 first, then trace how the Operator currently configures oauth2_proxy. Define the configuration surface needed for passthrough, including external OAuth provider settings, and verify that users can supply it without losing the existing authentication options.

Written by the indexing model from the issue text.