Add X-Total-Count on /repos/{owner]/{repo}/pulls API endpoint

[CirnoT]

This is required to allow Renovate Bot to fetch more than 10 PRs as seen here.

https://github.com/renovatebot/renovate/blob/f00846ee5d00ec72bed8806bb6d257262c2911e9/lib/platform/gitea/gitea-got-wrapper.ts#L32

The reason Renovate uses X-Total-Count for pagination is because this header is already present on repos list endpoint.

We should consider adding this header to all other endpoints that list, however for sake of quick fix I have made the change only to pulls endpoint for now.

[6543]

created #11114 to take track of the progress

[codecov-io]

Codecov Report

Merging #11113 into master will increase coverage by 0.06%.
The diff coverage is 31.88%.

@@            Coverage Diff             @@
##           master   #11113      +/-   ##
==========================================
+ Coverage   43.46%   43.53%   +0.06%     
==========================================
  Files         597      599       +2     
  Lines       84706    84825     +119     
==========================================
+ Hits        36821    36931     +110     
- Misses      43344    43348       +4     
- Partials     4541     4546       +5     

Impacted Files	Coverage Δ
models/issue_comment.go	46.68% <ø> (ø)
models/migrate.go	0.00% <0.00%> (ø)
models/migrations/migrations.go	3.50% <ø> (ø)
models/migrations/v136.go	0.00% <0.00%> (ø)
models/migrations/v137.go	0.00% <0.00%> (ø)
models/repo_language_stats.go	66.37% <ø> (ø)
modules/analyze/code_langauge.go	81.25% <ø> (ø)
modules/auth/repo_form.go	43.69% <ø> (ø)
modules/auth/sso/basic.go	50.81% <0.00%> (+7.15%)	⬆️
modules/git/repo_language_stats.go	66.17% <ø> (ø)
... and 38 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7eace60...4bad9dc. Read the comment docs.