[FEAT] Make an issue un-assignable through label

[Varun-Kolanu]

Is your feature request related to a problem? Please describe.
If maintainer wants an issue not to get assigned until needed, there is no barrier for bot to check that right now

Describe the solution you'd like

• The issue should not be able to claim if a label, say "unassignable" is on the issue
• The name of the label shall be chosen by the maintainer of repo
• The opening greeting comment to claim by the bot shall not be made and shall be made again when the label is removed
• The bot comments that the issue can't be claimed (this comment too shall be configurable)

Describe alternatives you've considered

• Alternative of commenting that issue can't be claimed can be directly skipping to do anything

[issue-assigner]

Comment '@issue-assigner claim' to get this issue assigned or '@issue-assigner abandon' to get this issue unassigned.