Temporary disable the test that causes a failure

This MR lays groundwork for chosen relation import
(https://gitlab.com/gitlab-org/gitlab/-/issues/425798) by updating
the relation tree restorer to allow importing of a single relation.

It also adds a service and class which will be called by an API endpoint
in a follow-up MR.

Changelog: changed

The http client has been always reused for send-url injector.

However, it changed recently when an ability to customize the
timeouts has been introduced. This MR caches http client and re-use it
for requests with same params.

In Google Cloud Storage, this is necessary to make it possible
to override the `Content-Type` via `Response-Content-Type` in
a presigned URL.

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/438535

Fix various warnings in cmd/gitlab-workhorse/upload_test.go

Changelog: other

In https://gitlab.com/gitlab-com/gl-infra/production/-/issues/17587,
our 500 error rates rose when incomplete multipart uploads were sent,
but this is really a client error, not a server error. This commit
downgrades that to a 400 error by doing an ugly string comparison in
`multipart.NextPart` to check for this condition.

Changelog: changed

Bump gitaly to v16.9.0-rc3 to ingest the updated service config added to
the client package with. This a retryPolicy which allows clients to
retry read-only RPCs if Gitaly is unavailable.

Changelog: changed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/143200