feat(migrations): add migration from Bitbucket Server (Data Center) (#39913)

Add migration from Bitbucket Server (Data Center). It has no issues,
wiki, milestones or releases, so those are not offered.

Migrated: git data with tags and LFS, and pull requests with state,
draft flag, merge commit/base, comments, approvals and inline code
comments including resolved threads. Multi-line ranges become a `Lines
a-b:` text prefix, as Gitea anchors code comments to one line. Auth is
an HTTP access token (personal, project or repository) or
username/password; 429 responses are retried.

Changes outside the downloader:
- `ReviewComment.Resolved`, applied by the uploader.
- The uploader creates a fork PR's head branch from its head commit when
already in the repository, instead of fetching the fork (Bitbucket keeps
it in `refs/pull-requests/<id>/from`; private forks need credentials).

Tested with unit tests, responses recorded from Bitbucket 10.5.1, and an
end-to-end migration. Independent of
https://github.com/go-gitea/gitea/pull/38607 (Bitbucket Cloud); the
service type enum value needs adjusting for whichever lands second.

---------

Co-authored-by: bircni <bircni@icloud.com>

services/convert/utils.go

@@ -40,6 +40,8 @@ func ToGitServiceType(value string) structs.GitServiceType {
 		return structs.CodebaseService
 	case "codecommit":
 		return structs.CodeCommitService
+	case "bitbucketserver":
+		return structs.BitbucketServerService
 	default:
 		return structs.PlainGitService
 	}

services/convert/utils_test.go

@@ -38,6 +38,8 @@ func TestToGitServiceType(t *testing.T) {
 		typ: "codebase", enum: 8,
 	}, {
 		typ: "codecommit", enum: 9,
+	}, {
+		typ: "bitbucketserver", enum: 10,
 	}}
 	for _, test := range tc {
 		assert.EqualValues(t, test.enum, ToGitServiceType(test.typ))