Make Data sink `Secret` configurable

[GenosseOtt]

Context

Our tool may run on any customer setup we cannot forsee.
Customers will bring their own system and tools to persist the collected Metrics Data in.
Customisation is therefore key.

Acceptance criteria

• End users can easily configure name of Secret holding their data source data
• Code doesnt hard-code Secret name

[mirzakopic]

I will do this now.

[mirzakopic]

Created PR #27

[maximiliantech]

I've created #25 to improve developer experience and get the users more easily started with the metrics-operator. After this PR is merged, feel free to iterate over it again and give feedback of what could be missing @GenosseOtt.