feat: Add support for API served via HTTPS when preferService: true

[weisdd]

Description

Noticed the issue #642, where a user complains that grafana-operator cannot manage grafana instance which exposes its API via https.

The reason for that is quite simple. - (r *ReconcileGrafana) getGrafanaAdminUrl always assumes that grafana instance serves HTTP when preferService: true.
Grafana itself supports http, https, h2, and socket (source). I think operator can only be guaranteed to work with http and https. h2 might be served via different protocols (HTTP / HTTPS), which makes its setup more complicated, so IMO it's better not support it at all. socket is definitely not the way operator is set up to work.

Relevant issues/tickets

Fixes: #642

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• This change requires a documentation update
• I have added tests that prove my fix is effective or that my feature works
• I have added a test case that will be used to verify my changes
• Verified independently on a cluster by reviewer

Verification steps

1. Generate cert:

$ openssl genrsa -out ca.key 2048
$ openssl req -x509 -new -nodes -days 365 -key ca.key -out ca.crt -subj "/CN=yourdomain.com"
kubectl create secret tls my-tls-secret --key ca.key --cert ca.crt

2. Deploy grafana with a spec similar to this:

spec:
  client:
    preferService: true
    timeout: 5
  config:
    server:
      cert_file: /etc/grafana/secrets/tls/tls.crt
      cert_key: /etc/grafana/secrets/tls/tls.key
      protocol: https
  dashboardLabelSelector:
  - matchLabels:
      app.kubernetes.io/instance: grafana-operator
  deployment:
    extraVolumeMounts:
    - mountPath: /etc/grafana/secrets/tls
      name: tls
    extraVolumes:
    - name: tls
      secret:
        secretName: my-tls-secret

For discussion

Please, let me know if you agree with the overall implementation. Once that is done, I can add some tests.

[HubertStefanski]

LGTM! Thanks for the contribution! @weisdd

[weisdd]

@HubertStefanski yw. I've just added some tests. Since there are plenty of factors that influence the end result and I tried to cover the preexisting cases as well, the code is slightly monstrous :) Please, feel free to make any changes if needed.