Add Gitlab retriever

[ruairi-wmf]

Description

Changes include

• Bugfix (non-breaking change that solves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking changes (change that is not backward-compatible and/or changes current functionality)

Closes issue(s)

Resolve #695

Checklist

• I have tested this code
• I have added unit test to cover this code
• I have updated the documentation (README.md and /website/docs)
• I have followed the contributing guide

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[codecov]

Codecov Report

Merging #696 (2189806) into main (d0b0c0d) will increase coverage by 0.10%.
The diff coverage is 93.84%.

@@            Coverage Diff             @@
##             main     #696      +/-   ##
==========================================
+ Coverage   88.94%   89.04%   +0.10%     
==========================================
  Files          58       59       +1     
  Lines        2713     2775      +62     
==========================================
+ Hits         2413     2471      +58     
- Misses        245      248       +3     
- Partials       55       56       +1     

Impacted Files	Coverage Δ
cmd/relayproxy/config/config.go	94.39% <ø> (ø)
cmd/relayproxy/service/gofeatureflag.go	56.61% <88.88%> (+1.36%)	⬆️
retriever/gitlabretriever/retriever.go	93.75% <93.75%> (ø)
cmd/relayproxy/config/retriever.go	100.00% <100.00%> (ø)

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[thomaspoignant]

Hello @ruairi-wmf thanks a lot for your pull request this is an awesome addition to GO Feature Flag.

I have looked at your PR and I have a few things that are not working.
I saw that you are not using the GitLab API to retrieve the files but the public raw access from the website.

From my early investigation, I think it could be great to use the API instead.
This API can help us doing what we want to do: https://docs.gitlab.com/ee/api/repository_files.html#get-raw-file-from-repository

I've done a quick CURL to test it and it seems to work perfectly.

curl --header "PRIVATE-TOKEN: <GITLAB TOKEN>" "https://gitlab.com/api/v4/projects/<REPOSITORY SLUG url encoded>/repository/files/<PATH OF YOUR file url encoded>/raw?ref=main"

A point of attention is to be sure that you URL encode both the repository slug and the path of your file.

• If your slug is thomas.poignant/test-go-feature-flag-provider is should be thomas.poignant%2Ftest-go-feature-flag-provider in the URL.
• If your path is testdata/flags.yaml it should be testdata%2Fflags.yaml in the URL.

[ruairi-wmf]

Thanks Thomas, I'll work on this tonight. Cheers!

[thomaspoignant]

Hey thanks again for the changes, I have left some new comments.
If you need any help or more information let me know.

[thomaspoignant]

Hey @ruairi-wmf I have used your PR to finish it with my remarks.
I will merge it Today and create a new version 1.10.0, you will be able to use the Gitlab retriever starting this version.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.1% Duplication