Fixes Opsgenie Create Alert failure when external image URL attached

[oneoneonepig]

Fixes issue: grafana/grafana#67177

When invoking the Opsgenie Create Alert API, the field detail seems to only accept string value, does not accept array in the value. If the value is an array, Opsgenie will respond with 422 Unprocessable Entity.

This fix converts the array to a comma joined string, and the following screenshot is a successfully sent alert with two images attached:

The following section is copied from the original issue:

What happened:

Cannot create Opsgenie Alerts with S3 image url attached

What you expected to happen:

Opsgenie Alerts are created and image URL are attached in details section

How to reproduce it (as minimally and precisely as possible):

1. Create a Grafana container
2. Create a Grafana Image Renderer container
3. Configure Grafana to use the Grafana Image Renderer by following the tutorial
4. Configure panels, alert rules, contact points, notification policies, etc.
5. Trigger an Grafana Alert
6. An image is created in the S3 bucket
7. Grafana log shows 422 Unprocessable Entity
8. No Opsgenie Alert is created

[CLAassistant]

All committers have signed the CLA.

[yuri-tceretian]

LGTM. I would really appreciate a test + some description (you can copy some from the issue).

[grobinson-grafana]

Could you also include your redacted screenshot from the GitHub issue in the description to show that you manually tested it too?

[oneoneonepig]

Hi @grobinson-grafana and @yuri-tceretian, I've added a brief description of the fix, the result screenshot, and copied a few sections from the original issue.

Let me know if anything else should be added or tested

[grobinson-grafana]

Thank you very much for your contribution! This will be released in Grafana 10. I will also look at how to backport it to 9.5 and 9.4.