Allow customized SMS message (resolves #197)

[strauss115]

What this PR does

Allows to fully customize the SMS message. Currently always "Grafana OnCall: Alert group #[NUMBER] ... from stack [STACK], integration: [INTEGRATION], alerts registered: [NUMBER]" is added. This PR removes the surrounding text and let's the user fully decide how the message looks like using the templates (as with other notification channels).

Which issue(s) this PR closes

Related to https://github.com/grafana/oncall/issues/197

Checklist

• [x] Unit, integration, and e2e (if applicable) tests updated
• [x] Documentation added (or pr:no public docs PR label added if not required)
• [x] Added the relevant release notes label (see labels prefixed w/ release:). These labels dictate how your PR will show up in the autogenerated release notes.

[CLAassistant]

All committers have signed the CLA.

[github-actions[bot]]

This pull request has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in 30 days if no further activity occurs. Please feel free to give a status update now, ping for review, or re-open when it's ready. Thank you for your contributions!

[github-actions[bot]]

This pull request has been automatically closed because it has not had activity in the last 30 days. Please feel free to give a status update now, ping for review, or re-open when it's ready. Thank you for your contributions!