It imitates a push to the secondary and verifies that the push
is redirected to the primary

Currently, we perform a request to Gitlab Rails that proxies
the request to primary

However, it causes timeouts on big pushes and consumes large
amount of memory. We can perform an HTTP request directly
from Gitlab Shell instead and stream the response to the user

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.

While gitlab-shell currently has a major version of v14, the module path
it exposes is not using that major version like it is required by the Go
standard. This makes it impossible for dependents to import gitlab-shell
as a dependency without using a commit as version.

Fix this by changing the module path of gitlab-shell to instead be
`gitlab.com/gitlab-org/gitlab-shell/v14` and adjust all imports
accordingly.

Changelog: fixed

This adds support for specifying host certificates via the
`host_cert_files` option and advertises the signed key to the
client. This acts similarly to OpenSSH's `HostCertificate` parameter:
gitlab-sshd attempts to match a host key to its certificate, and then
substitutes the matching host key with a certificate signed by a
trusted certificate authority's key.

This is the first requirement to supporting SSH certificates. This
will enable the client to trust the server if both trust a common
certificate authority. The `TrustedUserCAKeys` option will need to be
supported later for the server to trust all user keys signed by this
certificate authority.

Relates to https://gitlab.com/gitlab-org/gitlab-shell/-/issues/495

Issue: https://gitlab.com/gitlab-org/gitlab-shell/-/issues/576

ProxyHeaderTimeout must be small to avoid DoS risk

Let's make the value configurable and 500ms by default

- If an integer is specified, we assume that these are seconds
- A duration of format "500ms", "10s", "1m", etc... accepted

Before this change, the GitLab internal API could use a boolean
response field to indicate whether gitlab-shell should make
sidechannel connections go Gitaly. We now ignore that response field
and always use sidechannel connections.

If the GitLab API returns an allowed response with use_sidechannel set
to true, gitlab-shell will establish a sidechannel connection and use
SSHUploadPackWithSidechannel instead of SSHUploadPack. This is an
efficiency improvement.