Add custom description for OpsGenie

[nickbabkin]

Description

This change allows to set custom description field for OpsGenie alerts for more flexibility. Example use cases: where sensitive alert data must be masked from body before passing it to OpsGenie.

Description can be dynamically formatted from match by using {field} tags.

Checklist

• I have reviewed the contributing guidelines.
• I have included unit tests for my changes or additions.
• I have successfully run make test-docker with my changes.
• I have manually tested all relevant modes of the change in this PR.
• I have updated the documentation.
• I have updated the changelog.

[jertel]

Thanks for the contribution!

[nsano-rururu]

Would you like to add the following settings to schema.yaml?

opsgenie_description: {type: string}

[nickbabkin]

@nsano-rururu I've seen this was already added by you. Thanks for noting this!