CNTRLPLANE-2201: (auth): EP for generic external claims sourcing

[everettraven]

Adds an enhancement proposal to outline how we can add support for generically fetching user identity information from external sources to expose as claims in the direct external OIDC feature.

The main motivator for designing this feature is to make it easier for our customers to use the direct external OIDC configuration to work with use cases where not all the identity information for users of a cluster are presented as claims in a JWT.

We are also intentionally trying to approach this in a way that enables us to potentially contribute this logic back to the upstream Structured Authentication Configuration feature.

[openshift-ci-robot]

@everettraven: This pull request references CNTRLPLANE-2201 which is a valid jira issue.

Warning: The referenced jira issue has an invalid target version for the target branch this PR targets: expected the task to target the "4.22.0" version, but no target version was set.

In response to this:

Adds an enhancement proposal to outline how we can add support for generically fetching user identity information from external sources to expose as claims in the direct external OIDC feature.

The main motivator for designing this feature is to make it easier for our customers to use the direct external OIDC configuration to work with use cases where not all the identity information for users of a cluster are presented as claims in a JWT.

We are also intentionally trying to approach this in a way that enables us to potentially contribute this logic back to the upstream Structured Authentication Configuration feature.

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[openshift-ci]

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[openshift-ci]

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Test name	Commit	Details	Required	Rerun command
ci/prow/markdownlint	16f8821	link	true	/test markdownlint

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[everettraven]

I realized that this is missing some sort of TLS configuration layer for configuring the trust store of the webhook authenticator when making requests to this external source.

I'll hold off on adding any of that until there is some sort of general consensus on whether or not this approach is a reasonable path to continue pursuing.