bitbucketserver: Implement LoadPullRequestActivities

Created by: tsenart

This method is needed in the context of a8n to load all activity that happened in a PR. These activities will later be converted into ChangesetEvents.

Part of #6085

Created by: codecov[bot]

Codecov Report

Merging #6264 into master will increase coverage by 0.03%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #6264      +/-   ##
=========================================
+ Coverage   39.06%   39.1%   +0.03%     
=========================================
  Files        1185    1185              
  Lines       60402   60422      +20     
  Branches     5803    5803              
=========================================
+ Hits        23599   23628      +29     
+ Misses      34628   34620       -8     
+ Partials     2175    2174       -1

Impacted Files	Coverage Δ
internal/extsvc/bitbucketserver/client.go	44.75% <100%> (+4.95%)
cmd/frontend/graphqlbackend/graphqlbackend.go	56.72% <0%> (+1.75%)
internal/search/backend/horizontal.go	88% <0%> (+2%)