stable/redis-ha: add endpointAdditionalProperties for redis-ha and haproxy serviceMonitor

[trawler]

What this PR does / why we need it:

This PR enables setting additional flags for the prometheus-exporter and redis-ha serviceMonitors (see: EndpointSpec).
I opted to create an open nested object called endpointAdditionalProperties so that it's not necessary to explicitly expose all available endpoint settings.

Usage Example:

exporter:
  serviceMonitor:
    enabled: true
    endpointAdditionalProperties:
      relabelings:
        - sourceLabels: [__address__]
          targetLabel: foo
          replacement: bar

Special notes for your reviewer:

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• DCO signed
• Chart Version bumped
• Variables are documented in the README.md
• Title of the PR starts with chart name (e.g. [stable/mychartname])

[trawler]

@DandyDeveloper - can you trigger the tests?

[trawler]

tagging also @ssalaues: can you also help in triggering the tests for this PR? Thanks!

[DandyDeveloper]

Running the tests now :)

[trawler]

hmm looks like my commit was not signed. checking.

[trawler]

@DandyDeveloper I re-pushed with a signed commit. Sorry for the hassle!