feat(issue-12348): add validator index label to validator_statuses me…

[0xste]

What type of PR is this?
Feature

What does this PR do? Why is it needed?

• We don't currently have a way to retrieve validator index from prometheus
• It's useful for joining to various other metrics and, an example use-case is linking from a dashboard using the index as part of the target link i.e. https://beaconcha.in/validator/1101253 or joining with another metric to get additional metadata
• This has been added to the existing validator_statuses metric:
  • We don't have a redundant info metric & associated cardinality
  • We make a non-breaking change to the metric contract for the existing validator_statuses metric by adding an additional label

Which issues(s) does this PR fix?
Fixes #12348

Acknowledgements

• I have read CONTRIBUTING.md.
• I have made an appropriate entry to CHANGELOG.md.
• I have added a description to this PR with sufficient context for reviewers to understand this PR.