feat: add support for extra volume mounts for redis sentinel

[lsoica]

Description

Add the capability to mount extra volumes for the Redis Sentinel STS. One use case is the sentinel config file which is dynamically created at boot time so it needs write access to a file system.

Fixes #970

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• [x] Tests have been added/modified and all tests pass.
• [x] Functionality/bugs have been confirmed to be unchanged or fixed.
• [x] I have performed a self-review of my own code.
• [x] Documentation has been updated or added where necessary.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 44.78%. Comparing base (d121d86) to head (41cbb7c). Report is 75 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #994 +/- ## ========================================== + Coverage 35.20% 44.78% +9.58% ========================================== Files 19 20 +1 Lines 3213 2686 -527 ========================================== + Hits 1131 1203 +72 + Misses 2015 1413 -602 - Partials 67 70 +3 ```

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[lsoica]

@drivebyer could I have your review here please?

[drivebyer]

@drivebyer, could I have your review here, please?

@lsoica Sure, I will review this within today.