Contextual validation of reporter flags

[leodido]

Describe the feature or problem you’d like to solve

We have now --reporter (gh-pull-check,gh-pull-comment,gh-pull-review) and related flags (eg., --gh-pull-id, --gh-repo, etc.)

We need to build contextual validators so that --gh-* flags are only allowed when using the GitHub reporters.

Proposed solution

I propose to build 1 or more custom context validators on top of go-validator features.

Additional context

NONE