gha/add pr comment triggers

[swap357]

Currently, the GitHub Action workflows for building and testing llvmdev and llvmlite conda packages can only be triggered manually through the GitHub web interface using the workflow_dispatch event. This involves navigating to:

Actions tab → 'Run workflow' dropdown → Select branch and user inputs → Click 'Run'.:

Additionally, pull request triggers activate only when workflow files themselves change.
However, these triggers are inadequate for fully testing pull requests from forks, particularly when verifying the complete build chain (llvmdev → llvmlite).

To resolve this, the current PR introduces issue-comment event triggers using the github/command action. This setup enables maintainers with sufficient permissions (Numba developers) to invoke GitHub Action workflows directly through PR comments.

security and permissions:

• Only maintainers or contributors with write permissions can trigger workflows using the /gha llvmdev command in PR comments.
• The github/command action explicitly permits triggering workflows from forked repositories (allow_forks: true).
• Commands are strictly parsed and validated within the workflow to prevent unauthorized or unintended execution.

references:

• https://docs.github.com/en/actions/reference/events-that-trigger-workflows#issue_comment
• https://github.com/marketplace/actions/command-action

To achieve this, this PR adds issue-event triggers using github/command action. (github/command)

usage: 
[command] [keyword-args optional]
/gha llvmdev [platform='all', recipe='all']
/gha llvmlite [platform='all', llvmdev_run_id=''] 

llvmdev_run_id: Workflow ID obtained from the llvmdev run, required to retrieve artifacts.

[swap357]

example use of comment commands on a pull request here-
swap357#86
This also shows a test where a user with read-only access tries to run using comment command and fails.

[swap357]

closing this one
as it has 2 major issues -

every workflow we use issue_comment trigger on gets invoked on pr comment. there's no way to filter or check for what comment contains on issue_comment definition. So, a random comment can invoke all workflows, check for command and not proceed. But workflows showing up on Actions tab will grow in order of comments*workflows .
We also need to link workflow triggered to pr final checks.