feat(ci/lint): Allow printing merged yaml result

Description

This adds --print-merged flag to glab ci lint command to output merged yaml that is returned by gitlab api. When enabled - yaml is output to stdout, and validity message to stderr. This was output can be piped and processed as valid yaml.

Related Issues

Resolves #1349 This was first posted in !1287 (closed)

How has this been tested?

Unit test included. Also tested on live Gitlab ce v16 instance and real projects. Changed only single command and it's tests, this should not affect any other areas.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap