[TASK] Add GitHub Action to support "slash commands" for adding or removing sub-issues

[andyatmiami]

Description

With the recent community agreement for Notebooks 2.0 to use an Epic -> Feature -> Issue breakdown to categorize and track work, we need better tooling to enable linking issues.

Presently, the issue author of an [EPIC] or [FEATURE] will include the list of child GH issues in the issue description. This creates a "manual touch point" in that someone else writing up a child issue needs to "ping" the parent issue owner to have them update the description. It also creates potential (minor but annoying) inconsistencies in how child issues are tracked:

• - [] <issue ref>
• - <issue ref>

To promote good organizational practices - while also reducing the need to human-to-human communication - we want to support "slash" commands through GitHub actions to be able to manage/maintain the child issue list through GitHub's sub-issue feature. This will enable child issue authors (or any EPIC owners) to easily add/remove issues in a self-service fashion.

Commands to implement:

• /add-sub-issue
• /remove-sub-issue

Please refer to the slack discussion where behavior/implementation details were discussed.
- Included in that slack discussion is a link to a "proof of concept" solution I worked on

Acceptance Criteria

• Logic is implemented as a GitHubAction
• /add-sub-issue #123 would add Issue #123 as a sub-issue to the issue in which the command was added as a comment
• /remove-sub-issue #123 would remove Issue #123 as a sub-issue from the issue in which the command was added as a comment
• Multiple issue references can be provided to /add-sub-issue and /remove-sub-issue command

[andyatmiami]

/area v2
/area repo-setup

[andyatmiami]

/assign