MM-51310 Implement OAuth2 Authentication (reopened)

[mickmister]

Summary

Reopening of #928

This PR implements the OAuth2 authentication in the Jira Plugin.

Ticket Link

Fixes https://mattermost.atlassian.net/browse/MM-51310

Screenshots

Install OAuth2 using setup flow
Whenever the system admin runs the command /jira setup, the setup flow to configure OAuth2 client would look something like this:

[codecov-commenter]

Codecov Report

Patch coverage: 3.84% and project coverage change: -1.29% ⚠️

Comparison is base (7c1b921) 31.16% compared to head (dc58dd5) 29.88%.
Report is 4 commits behind head on master.

❗ Current head dc58dd5 differs from pull request most recent head 6df75bc. Consider uploading reports for the commit 6df75bc to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #949      +/-   ##
==========================================
- Coverage   31.16%   29.88%   -1.29%     
==========================================
  Files          50       52       +2     
  Lines        7409     7734     +325     
==========================================
+ Hits         2309     2311       +2     
- Misses       4909     5231     +322     
- Partials      191      192       +1     

Files Changed	Coverage Δ
server/client.go	8.04% <0.00%> (ø)
server/command.go	15.39% <0.00%> (-0.63%)	⬇️
server/instance.go	89.47% <0.00%> (-10.53%)	⬇️
server/instance_cloud.go	6.43% <0.00%> (-0.08%)	⬇️
server/instance_cloud_oauth.go	0.00% <0.00%> (ø)
server/instance_server.go	18.88% <0.00%> (-0.43%)	⬇️
server/kv.go	17.91% <0.00%> (-0.36%)	⬇️
server/plugin.go	11.56% <0.00%> (-0.05%)	⬇️
server/setup_flow.go	0.00% <0.00%> (ø)
server/user.go	26.70% <ø> (ø)
... and 6 more

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[raghavaggarwal2308]

@mickmister @DHaussermann Fixed issue: Comment webhook notification not working in the following PR because I didn't have the access to push the code in this PR: #953

[BlueSky-fur]

🥳 2 more to go to make work easier again! 😄

[Jawknee-cache]

Any update on when this issue will be resolved?

#945

[hanzei]

@mickmister What are the next steps on this PR? Is it ready for QA testing?

[mickmister]

@hanzei Yes it's ready for QA testing. Discussing in Brightscout channel next steps for testing https://community.mattermost.com/core/pl/ng4tz8stp7ynucq7bgrp9edj7y

[AayushChaudhary0001]

Tested and approved, LGTM

• This PR is tested and the setup of OAuth 2.0 is completely working fine.
• This PR was tested on Jira cloud latest version and MM version 7.8.2.
• During the testing, the notifications were also found working.

[mickmister]

During the testing, the notifications were also found working.

@DHaussermann It seems the Jira comment events are fixed now. Merging the PR, and we can cover any more regression testing in the release testing.