MM-37922 - Add support for Chimera to GitLab plugin

[Szymongib]

Summary

Chimera Proxy is an easier way to connect plugins to external OAuth providers. It provides pre-registered OAuth applications allowing users to skip the app registration step.

Design doc: https://docs.google.com/document/d/1a1AzNwxrqA5bcVonZOVu94b4vV6tT4U7fDIKBwf_aUI/edit

This PR makes it possible to connect GitLab Plugin via Chimera. It introduces:

• New config option, whether to use pre-registered application (Chimera)
• Changes validation if connection through Chimera is enabled
• Creates different OAuth config when using Chimera

Ticket Link

https://mattermost.atlassian.net/browse/MM-37922

[codecov]

Codecov Report

Merging #255 (31958cf) into master (6755649) will decrease coverage by 0.39%.
The diff coverage is 13.33%.

@@            Coverage Diff             @@
##           master     #255      +/-   ##
==========================================
- Coverage   39.78%   39.38%   -0.40%     
==========================================
  Files          16       16              
  Lines        1757     1795      +38     
==========================================
+ Hits          699      707       +8     
- Misses        984     1011      +27     
- Partials       74       77       +3     

Impacted Files	Coverage Δ
server/plugin.go	11.65% <0.00%> (-1.71%)	⬇️
server/configuration.go	32.55% <75.00%> (+17.17%)	⬆️

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[iomodo]

LGTM! Thanks @Szymongib

[hanzei]

Great work 👍

[hanzei]

Nit pick: Unrelated change

[hanzei]

@DHaussermann See mattermost/mattermost-plugin-github#467 (comment) of for the tests steps.

[jasonblais]

@DHaussermann Kind reminder to help with QA review. Similar to mattermost/mattermost-plugin-github#467, we'd like to get ready to ship the changes ahead of the Oct 13 launch, and would be happy to help where we can.

As an aside, I wonder if Demansol would be equipped to help test the changes here or at mattermost/mattermost-plugin-github#467?

[Szymongib]

To align with the implementation of the GitHub plugin and support both v5 and v6 servers I have added a fallback to Chimera env var. For more context see: mattermost/mattermost-plugin-github#467 (comment)

[DHaussermann]

@Szymongib I have done a round of testing on this and it's looking good 👍

• Regression tested existing oAuth config
• Tested authentication using the proxy on a cloud server
• Tested to ensure the plugin can fail over to the environment variable when no value in the config

The only feedback I have is to add the same sentence to the config option as we have on GitHub
"This setting is intended to be used with Mattermost Cloud instances only."

[Szymongib]

@DHaussermann I have aligned the setting text with the GitHub plugin.

[DHaussermann]

Tested and passed

• Functional testing completed
• I know see the text change on config as expected

Thanks @Szymongib

LGTM!