Add NamespaceApi.getNamespace(idOrPath) using /namespaces/:id

[ddoleye]

Just like GroupApi.getGroup(idOrPath) and ProjectApi.getProject(idOrPath),
getNamespace(idOrPath) method has been added to NamespaceApi as well.

See: https://docs.gitlab.com/ee/api/namespaces.html#get-namespace-by-id

[jmini]

This looks like a good addition, but according to our branching model it should target main. See #926 (comment)

I will then cascade the change to 6.x.

[jmini]

I have changed the target branch of this PR.

git rebase --onto origin/main origin/6.x
git push --force

Be aware that your local branch 6.x has diverged with the one on the GitHub.
If you need to do one more changes on this branch:
I recommend to use git fetch and git reset --hard origin/6.x before doing any local modifications instead of git pull.