Add ContactPointRef and ContactPointSelector fields to rulegroups.alerting.grafana.crossplane.io

[holgerjh]

Description of your changes

This PR adds reference and selector support to the alerting.grafana.crossplane.io RuleGroup resource. With that, one can use a contact point selector like follows:

apiVersion: alerting.grafana.crossplane.io/v1alpha1
kind: RuleGroup
metadata:
  name: "example"
spec:
  forProvider:
    name: "example"
    ...
    rule:
      - notificationSettings:
        - contactPointSelector:
            matchLabels:
              label: value

I have:

• [X] Read and followed Crossplane's [contribution process].
• [X] Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

Tested locally with kind cluster and k8s objects against grafana cloud API.

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 48 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@803d418). Learn more about missing BASE report.

Files	Patch %	Lines
apis/alerting/v1alpha1/zz_generated.resolvers.go	0.00%	32 Missing :warning:
apis/alerting/v1alpha1/zz_generated.deepcopy.go	0.00%	16 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #166 +/- ## ====================================== Coverage ? 0.00% ====================================== Files ? 201 Lines ? 29579 Branches ? 0 ====================================== Hits ? 0 Misses ? 29579 Partials ? 0 ``` | [Flag](https://app.codecov.io/gh/grafana/crossplane-provider-grafana/pull/166/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=grafana) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/grafana/crossplane-provider-grafana/pull/166/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=grafana) | `0.00% <0.00%> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=grafana#carryforward-flags-in-the-pull-request-comment) to find out more.

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