GroupByVariable: Sync label to URL

[bfmatei]

This PR syncs labels of the values in the group by variable similarly to what we introduced for ad-hoc filters.

Unlike ad-hoc filters, group by is a little bit more of a special case hence the reason why some of the changes below. The solution might not be optimal, so please feel free to suggested changes and improvements.

Changes:

• introduced custom URL sync handler
  • it works similarly to the ad-hoc filters one
  • this is needed as currently we use the one multi-value variable one and we don't want this changed for all variables
• the state of group by variable received a new prop: urlOptions where we store the options that we received via the URL
  • when fetching the options, we concat the urlOptions with what we receive from the datasource (or with the defaultOptions)
  • this is needed because of validateAndUpdate and also because changeValueTo is sometimes called without the texts array
• we leverage the fact that the value of the MultiSelect can be a SelectableValue hence we can enforce the labels on values
• moved some common functions to variables utils

The gotcha: let's assume we pass a value via URL. This value is not returned by the datasource hence we consider it a custom value. We then remove the custom value using the UI button. When opening the dropdown again, the custom value can be found in the dropdown. Is this a behavior we want to keep? You can also see this behavior in the demo below.

PS: tests will come in a subsequent push. For the time being I'm trying to decide upon edge cases.

Demo:

https://github.com/grafana/scenes/assets/9215315/54ca86b9-9db9-4ac1-ac8e-144a8cd576f2

📦 Published PR as canary version: 4.20.0--canary.705.9031064372.0

:sparkles: Test out this PR locally via: ```bash npm install @grafana/scenes@4.20.0--canary.705.9031064372.0 # or yarn add @grafana/scenes@4.20.0--canary.705.9031064372.0 ```

[grafanabot]

:rocket: PR was released in v4.20.0 :rocket: