T7445: added prs conflict check caller workflows

[kumvijaya]

Change summary

Added pr conflict checker caller workflow added which will be triggered when PR head changes.
Added open prs conflict checker caller workflow which will be triggered when PR base changes.
This will be triggered when push happens to current, will be checking open PRs of current and add the conflict label if it has conflict.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

https://vyos.dev/T7445

Related PR(s)

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly