Feature Flag YAML definition

TL;DR

We will make each feature flag explicit and well documented as part of YAML definition stored in repository.

Related to: #30228 (closed) and !31610 (closed). They contain all prior discussions.

Proposal

1. We introduce bin/feature-flag to easily add feature flag
2. We store each feature flag as YAML
3. We introduce feature flag types
4. We strongly validate feature flag usage, default_enabled/type

YAML

---
name: ci_project_subscriptions
introduced_by_url: 
rollout_issue_url: 
group: 'group::verify'
type: licensed
default_enabled: true

bin/feature-flag

➜  gitlab-rails bin/feature-flag my-feature-flag              
>> Please specify the type of your feature flag:

    development      Short lived, used to enable unfinished code to be deployed

?> development
>> Please specify the group introducing feature flag, like `group::apm`:

?> group::apm 
>> Open this URL and fill the rest of details:
https://gitlab.com/gitlab-org/gitlab/-/issues/new?issue[title]=%5BFeature+flag%5D+Rollout+of+%60my-feature-flag%60%60&

>> Paste URL here, or enter to skip:

?> 
create config/feature_flags/development/my-feature-flag.yml
---
name: my-feature-flag
introduced_by_url:
rollout_issue_url:
group: group::apm
type: development
default_enabled: false