Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/438521

* Disable false-positive linter finding
* Extract ServeExisting logic into smaller functions

Changelog: other

- Needed by Web IDE since extensions run in sandboxed iframe

Changelog: fixed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/80374

As mentioned in https://pkg.go.dev/encoding/json#Marshal:

The "omitempty" option specifies that the field should be omitted from
the encoding if the field has an empty value, defined as false, 0, a
nil pointer, a nil interface value, and any empty array, slice, map,
or string.

As a result, for 0-byte files, the JSON output of the ZIP metadata
omits the `crc` and `size` fields. Since we always expect this to be
generated, drop the `omitempty` entries from `crc`, `size`, and
`zipped` fields.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/460378

Changelog: fixed

https://gitlab.com/gitlab-org/gitlab/-/issues/356868

Changelog: added

Gitaly requires GlRepository to be set in calls that invoke the
hooks. Rails' internal API is disabled in tests so the actual
value doesn't matter though. Fill the value anyway so the
validation passes when sending requests to Gitaly.

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