add priority support for opsgenie

[patrick-tang]

To make opsgenie support priority input.

[patrick-tang]

Hi,
This is the first time I'm making a pull request. Apologize if I'm doing anything wrong and I'll improve it.
Kindly please let me know how I can put a proper description and what else I'm missing in my PR.
We're trying to integrate ArgoCD with OpsGenie, however noticed priority is not picked up by notification to send to OpsGenenie, so would expect the priority support can be added.
Thanks,
Patrick

[patrick-tang]

Forgot to update my git configuration. Still using my old company email, so signed off doesn't my commit email.
Let me delete and recreate the PR.