Implements the feature requested in gitlab-org/gitlab#19672

This requires the internal api counterpart in gitlab-org/gitlab!36302 to
be merged first.

It can be used as follows:
```
censored@censored-VirtualBox:~/git/gitlab$ ssh git@gitlab-2004 personal_access_token
remote:
remote: ========================================================================
remote:
remote: Usage: personal_access_token <name> <scope1[,scope2,...]> [ttl_days]
remote:
remote: ========================================================================
remote:

censored@censored-VirtualBox:~/git/gitlab$ ssh git@gitlab-2004 personal_access_token newtoken read_api,read_repository 30
Token:   aAY1G3YPeemECgUvxuXY
Scopes:  read_api,read_repository
Expires: 2020-08-07
```

Both git-upload-pack and git-receive-pack services inspect the
GIT_PROTOCOL environment transferred via SSH in order to decide which
protocols are supported by a given client. Currently, we don't use the
environment variable at all, though, but instead forward the GitProtocol
field of the access verification response.

Improve this by passing on the GIT_PROTOCOL environment variable
provided by the client as-is.