Add expires_at field when creating project deploy keys

[heidiberry]

Fixes https://github.com/xanzy/go-gitlab/issues/2020

The code in this area was a bit confusing. There are two similar endpoints:

• https://docs.gitlab.com/ee/api/deploy_keys.html#add-deploy-key
• https://docs.gitlab.com/ee/api/deploy_keys.html#add-deploy-key-for-a-project

The code linked to the documentation for the first one and was named like it was the first one, but it was actually the second one.

I've updated the documentation link, but adding support for the first one will be difficult in future given it's name is already taken.