Add opsgenie provider

[Alan01252]

This pull request adds opsgenie as a provider and allows extra metadata to be configured against an alert.
This is useful when you're running multiple clusters and want to distinguish where the alert has come from ( for example dev/staging/prod environments )

Opsgenie has been added as a receiver and this has been tested locally.
Opsgenie makes use of the extra metadata in order to route requests to appropriate slack channels etc.

[Alan01252]

As discussed I've removed the extraMetadata functionality and confirmed summary field appears in opsgenie if added to the Alert manifest.

[Alan01252]

Interesting this is failing. I was debating whether I need to check-in the output of make manifest but this causes a rather larger diff than I was expecting so I assumed I was missing something. I could just manually add opsgenie to the crd which would resolve this..

[stefanprodan]

You need to run make generate api-docs and commit the changes.

[Alan01252]

You need to run make generate api-docs and commit the changes.

There are no changes to the API docs when running that command, the build was failing because of a diff in the CRD docs. If I run make manifests locally I get a much larger diff than expected.

The current commit should be all that's required to make the build pass.

[stefanprodan]

LGTM

Thanks @Alan01252 🏅