Allow printing merged yaml in `ci lint`

Problem to solve

As a developer with a complex .gitlab-ci.yml, I want to be able to view merged .gitlab-ci.yml as returned by gitlab api on validation, so I can check that extends, yaml anchors, etc. are used correctly and are expanded as intended.

Proposal

Add --print-merged flag to glab ci lint that, once enabled, will:

• output MergedYaml field from the api response.
• output other messages from command to stderr so that result yaml can be piped further (for example glab ci lint --print-merged | bat -l yaml)

I already have working code in my fork, opening this issue before MR to comply with repository rules.

Further details

Links / references