[stable/redis-ha] Add additional ports for haproxy

[TheBigLee]

What this PR does / why we need it:

This PR adds a new field haproxy.additionalPorts that allows configuring additional ports for haproxy. This is useful when using a custom haproxy config with additional frontends that need to be exposed.

Which issue this PR fixes

None

Special notes for your reviewer:

Checklist

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

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

[TheBigLee]

@DandyDeveloper Is there anything else required from my side to move this forward?