redis-ha: make readiness and liveness probes optional, add startup probe

[bootc]

What this PR does / why we need it:

Allow disabling the readiness and liveness probes on both Redis and Sentinel. This can be useful if issues are encountered with the probes. Also add a startup probe which can better handle the sometimes lengthy process of loading a large database and subsequently refreshing the local copy from the primary / master server.

I have previously worked around this by adding a post-renderer to modify the generated manifests, but I'd like to do it better.

Which issue this PR fixes

None.

Checklist

• [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])