rds.cluster: add ability to auto-generate password in referenced secret

[ravibagri5]

Description of your changes

Adds the ability to auto-generate password for the rds cluster. This was already added for the RDS instance in the PR #628. Users need to opt-in by setting autoGeneratePassword as true while creating RDS cluster and should have a Secret reference. It will create if the referenced Secret does not exist and it will populate if it doesn't have the password for the given key.

I have:

• [x] Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

Unit tests, local end-to-end test.

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