Add API to get PR by base/head (#29242)

Closes https://github.com/go-gitea/gitea/issues/16289

Add a new API `/repos/{owner}/{repo}/pulls/{base}/{head}` to get a PR by
its base and head branch.

routers/api/v1/api.go

@@ -1225,6 +1225,7 @@ func Routes() *web.Route {
 							Delete(bind(api.PullReviewRequestOptions{}), repo.DeleteReviewRequests).
 							Post(bind(api.PullReviewRequestOptions{}), repo.CreateReviewRequests)
 					})
+					m.Get("/{base}/*", repo.GetPullRequestByBaseHead)
 				}, mustAllowPulls, reqRepoReader(unit.TypeCode), context.ReferencesGitRepo())
 				m.Group("/statuses", func() {
 					m.Combo("/{sha}").Get(repo.GetCommitStatuses).