feat: Add NetworkType + IPDiscovery to ReplicationGroup

[michaelhtm]

Issue #2565

Description of changes:

• support NetworkType + IPDiscovery
• add unit tests

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[michaelhtm]

/test elasticache-kind-e2e

[ack-prow]

@michaelhtm: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
elasticache-verify-attribution	4670044	link	false	/test elasticache-verify-attribution

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[rushmash91]

qq: why does it need to be immutable?

[rushmash91]

looked it up..

[rushmash91]

/lgtm

[ack-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: michaelhtm, rushmash91

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