fix(gitlab): correctly handle GitLab MR events

[corrieriluca]

Changes

Bug fixes ⚠️

• Use object_attributes.iid for MR id instead of object_attributes.id since GitLab API needs merge_request_iid in all MR related API calls.

Enhancements

• Use the ListMergeRequestDiffs API call instead of the deprecated GetMergeRequestChanges.
• Use pagination with the ListMergeRequestDiffs API call.
• Handle reopen and merge action in webhook handler to respectively create and remove TerraformPullRequest objects.

Changes tested in GitLab SaaS 16.6

[codecov-commenter]

Codecov Report

Merging #178 (e06fb7b) into main (0d0cd65) will increase coverage by 0.10%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main     #178      +/-   ##
==========================================
+ Coverage   70.50%   70.61%   +0.10%     
==========================================
  Files          28       28              
  Lines        1926     1926              
==========================================
+ Hits         1358     1360       +2     
+ Misses        492      490       -2     
  Partials       76       76              

Files	Coverage Δ
internal/webhook/gitlab/provider.go	75.00% <100.00%> (+3.33%)	⬆️

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