Add new runner api support

[RicePatrick]

This MR adds support for the new user-based Gitlab Runner API, documented here: https://docs.gitlab.com/ee/api/users.html#create-a-runner

It's worth noting that it's in the users.go file even though it's creating a runner, because it's in the /user api space. I could be convinced to move it to the runners.go file pretty easily, but figured the default stance was to align to the upstream API space.

It's also worth noting that there is no /users/runner DELETE api, because it uses the existing APIs within runners.go to delete by ID.

Let me know if you have any questions! I'd appreciate cutting a new version after merging this MR so that I can start working on the tf provider resource :)

[RicePatrick]

@svanharmelen - This should be ready for review when you have a moment!

[metanerd]

Thank you @PatrickRice-KSC ! I just found your PR, because I am also looking forward to this feature in the terraform provider. To my knowledge working on the tf provider resource should already work with go mod replace. With this message, it is not my intention to put pressure on this or the tf provider PR.

Edit: OK, I understand now. Because of the changes that might still be needed. Nvm, ignore me.

[RicePatrick]

@metanerd

I just found your PR, because I am also looking forward to this feature in the terraform provider. To my knowledge working on the tf provider resource should already work with go mod replace. With this message, it is not my intention to put pressure on this or the tf provider PR.

You're right, I could actually compile the library, use it locally, the develop that way. Sander is usually pretty quick on the review though, and between that an Renovate it means it's usually just cleaner to write it cleanly the first time :) . No worries on the pressure on the provider, I know it's a highly anticipated resource!

[svanharmelen]

Hey @PatrickRice-KSC 👋🏻 Take a look to the things I changed in the last commit and please try to follow the same style and pattern next time 😉

Thanks!

[RicePatrick]

Will do; thanks @svanharmelen !