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>

templates/swagger/v1-openapi3.generated.json

@@ -7988,7 +7988,8 @@
               "onedev",
               "gitbucket",
               "codebase",
-              "codecommit"
+              "codecommit",
+              "bitbucketserver"
             ],
             "type": "string",
             "x-go-name": "Service"

templates/swagger/v1-swagger.generated.json

@@ -31829,7 +31829,8 @@
             "onedev",
             "gitbucket",
             "codebase",
-            "codecommit"
+            "codecommit",
+            "bitbucketserver"
           ],
           "x-go-name": "Service"
         },