refactor(SceneByVariableRepeaterGrid): Pull out "groupBy" logic out of the class

[grafakus]

✨ Description

Related issue(s): is blocked by https://github.com/grafana/explore-profiles/pull/108

This PR aims at making SceneByVariableRepeaterGrid more generic.

We achieve this:

• by removing all the custom logic related to the "groupBy" variable from this class
• by creating a new SceneLabelValuesGrid class, responsible for rendering the panels grid after the user has selected a "groupBy" value
• by moving all the logic into the new class

We have a bit of duplicated code, but it's temporary. In the next PR, we'll start customizing SceneLabelValuesGrid so that the "Comparison" flow is easier to understand for the user (currently it's not obvious for users to understand how they can compare 2 label values).

📖 Summary of the changes

See diff tab for specific comments

🧪 How to test?

• The build should pass
• All panels grid should render and work as before

[github-actions[bot]]

Unit test coverage

Lines	Statements	Branches	Functions
	12.17% (470/3860)	9.61% (138/1435)	9.16% (108/1178)