Add Pipeline Variables endpoint

[capripot]

Gitlab.pipeline_variables(12, 3)
# => [#<Gitlab::ObjectifiedHash:70126709383040 {hash: {"variable_type"=>"env_var", "key"=>"FOO", "value"=>"bar"}}]

https://docs.gitlab.com/ee/api/pipelines.html#get-variables-of-a-pipeline