Implement option to override id of replicaSet (see #1650)

[1st8]

Closes #1650

Summary:

This PR adds a new optional string property in OverrideReplicaSet, which can be used to set the _id of the generated replicaSet config. See #1650 for details of the use case.

I implemented it as a new property in order to not break any existing setups or the default behaviour.

The change has been verified with a test and also by deploying the modified operator to our environment, where it achieves the desired result.

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