authproxy connector: add support for specifying group header separator

[a-buck]

Preflight Checklist

• [x] I agree to follow the Code of Conduct that this project adheres to.
• [x] I have searched the issue tracker for an issue that matches the one I want to file, without success.

Problem Description

This issue proposes making the group header splitter configurable so a value other than , can be used.

This is motivated as the groups header is not universally a ,. For example the default group separator in mod_auth_mellon is ;.

To give a very specific example of how it would be useful, this would enable use of the dex integration for SSO in argocd, where the groups are not separated with a comma.

# https://argo-cd.readthedocs.io/en/stable/operator-manual/user-management/#dex

argo-cd:
  dex:
    enabled: true

  configs:
    cm:
      dex.config: |
        connectors:
        - type: authproxy
          id: corpproxy
          name: corpproxy
          config:
            userHeader: SSO-User
            groupHeader: SSO-Groups
            groupHeaderSeparator: ";"

Proposed Solution

• groupHeaderSeparator is added as a configurable option for the authproxy connector
• the change is backwards compatible - maintaining , as a default

Alternatives Considered

No response

Additional Information

please see https://github.com/dexidp/dex/pull/3745 for example implementation, happy to revive the PR if there is feedback.