Add developer documentation to sshd package

See merge request https://gitlab.com/gitlab-org/gitlab-shell/-/merge_requests/683

Merged-by: Ash McKenzie <amckenzie@gitlab.com>
Approved-by: Alejandro Rodríguez <alejandro@gitlab.com>
Approved-by: Oscar Tovar <otovar@gitlab.com>
Approved-by: James Fargher <proglottis@gmail.com>
Approved-by: John Cai <jcai@gitlab.com>
Approved-by: Amy Qualls <aqualls@gitlab.com>
Approved-by: Ash McKenzie <amckenzie@gitlab.com>
Co-authored-by: Igor Drozdov <idrozdov@gitlab.com>

MustStrictWhiteListPolicy panics when configured incorrectly. So here we
use the error returning version instead.

Changelog: added

Instead of interpreting the configuration for every new connection, we
can rely on a closure to simplify the proxy handler path. This is more
similar to how the provided MustStrictWhiteListPolicy works which will
be added in a later commit.

Gitaly started to require `Repository` information for certain
commands like upload-pack, upload-archive, and receive-pack.

However, gitlab-shell tests are sending `git_object_directory` and
`git_alternate_object_directories` that do not exist.

To fix the tests, we set those to an existing directory in the
created repository.

Add developer documentation to command package

See merge request https://gitlab.com/gitlab-org/gitlab-shell/-/merge_requests/594

Merged-by: Patrick Bajao <ebajao@gitlab.com>
Approved-by: Patrick Bajao <ebajao@gitlab.com>
Co-authored-by: Igor Drozdov <idrozdov@gitlab.com>

This commit also excludes gitlab-shell from dependencies:

Gitaly specifies Gitlab Shell as a dependency as well in order
to use gitlabnet client to perform API endpoints to Gitlab Rails.
As a result, Gitlab Shell requires Gitaly -> Gitaly requires an
older version of Gitlab Shell -> that version requires an older
version of Gitlab Shell, etc. Let's use exclude to break the
chain earlier