incident.io notifier: add metadata field with templating support

[leonore]

Allow users to send custom key-value metadata with alerts, with full Go template rendering support (e.g. {{ .CommonLabels.severity }}).

Pull Request Checklist

Please check all the applicable boxes.

• Please list all open issue(s) discussed with maintainers related to this change
  • There is no issue where this is formally requested, but it was mentioned here, and through other threads.
• Is this a new Receiver integration?
  • I have already tried to use the Webhook Receiver Integration and 3rd party integrations before adding this new Receiver Integration
• Is this a bugfix?
  • I have added tests that can reproduce the bug which pass with this bugfix applied
• Is this a new feature?
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  • My changes do not break the existing cluster messages
  • My changes do not break the existing api
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Which user-facing changes does this PR introduce?

[ENHANCEMENT] The incident.io notifier now supports metadata with templating.