Cleanup campaign connection resolver

Created by: eseliger

This PR does two things (reviewable in separate commits):

• Move campaigns.go to two separate files, the connection and the node, like we do with most other entities now, and add an additional test that pagination works properly
• Apply the cursor pagination pattern from changeset specs to campaigns, which makes them more aligned and gives us a performance benefit when looking through a bigger list of campaigns

Also: Enjoy our variety of different arguments accepted to the same connection resolver :)

Created by: codecov[bot]

Codecov Report

Merging #13029 into main will increase coverage by 0.01%. The diff coverage is 62.22%.

@@            Coverage Diff             @@
##             main   #13029      +/-   ##
==========================================
+ Coverage   51.12%   51.13%   +0.01%     
==========================================
  Files        1469     1470       +1     
  Lines       82446    82463      +17     
  Branches     6683     6845     +162     
==========================================
+ Hits        42150    42171      +21     
+ Misses      36687    36683       -4     
  Partials     3609     3609              

Flag	Coverage Δ
#go	52.39% <71.79%> (+0.02%)
#integration	24.58% <0.00%> (-0.02%)
#storybook	14.84% <0.00%> (-0.01%)
#typescript	47.67% <0.00%> (-0.02%)
#unit	34.42% <0.00%> (-0.01%)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cmd/frontend/graphqlbackend/campaigns.go	0.00% <ø> (ø)
...nterprise/internal/campaigns/resolvers/campaign.go	81.96% <ø> (ø)
...terprise/internal/campaigns/resolvers/changeset.go	64.81% <0.00%> (-1.23%)
...src/enterprise/campaigns/list/CampaignListPage.tsx	38.09% <0.00%> (-4.02%)
web/src/enterprise/campaigns/list/backend.ts	16.66% <0.00%> (-2.39%)
...nterprise/internal/campaigns/resolvers/resolver.go	70.91% <60.00%> (+0.43%)
...nternal/campaigns/resolvers/campaign_connection.go	86.20% <86.20%> (ø)
web/src/tree/TreeRoot.tsx	79.31% <0.00%> (-1.73%)
.../internal/codeintel/resolvers/graphql/locations.go	87.27% <0.00%> (+1.81%)
... and 2 more