Allow configuration of returned groups via authproxy connector

[seuf]

Overview

Added groupHeader and groups configuration for authproxy connector.

Example :

connectors:
      - type: authproxy
        id: http-proxy
        name: http-proxy
        config:
          userHeader: X-Forwarded-User # default is X-Remote-User
          groupHeader: X-Forwarded-Group # default is X-Remote-Group
          staticGroups:
            - admin

What this PR does / why we need it

This allow users to configure argocd dex to use authproxy connector and specify the users default group and the HTTP header to read to get the group.

[nabokihms]

Hello, @seuf. Thank you for your intention to work on this feature.

1. I left a comment. We need to figure out the right approach before moving further.
2. Would you mind adding the note that authproxy now supports groups to the README.md?

   dex/README.md

   Line 78 in 716eef8

   | [AuthProxy](https://dexidp.io/docs/connectors/authproxy/) | no | no | no | alpha | Authentication proxies such as Apache2 mod_auth, etc. |

[seuf]

Hi @nabokihms I've updated this PR.

You can now configure both userHeader and groupHeader (defaults to X-Remote-User and X-Remote-Group).
You can also configure a list of groups (can be empty)
if http group header is present then it is appended to the configured group list.

I've tested it on my argocd dex deployment, and it works fine.

[nabokihms]

Looks good! Thank you for your work. There are some comments left. Please look through them when you have time.

And one more thing. I would like you to also make changes to the module documentation to inform users about the feature you have made.
https://github.com/dexidp/website/blob/main/content/docs/connectors/authproxy.md

[seuf]

Hi @nabokihms I've applied the changes you suggested. PR for the website documentation :
dexidp/website#109

[seuf]

Hi @nabokihms Do you have time this week to review this again ?