Make Topic Name an immutable field

[michaelhtm]

Issue #2196

Description of changes:

• Make topic name immutable
• Add e2e test checking for error after topic name change

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

[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
sns-verify-attribution	89d2e46	link	false	/test sns-verify-attribution

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[a-hilaly]

Nice! Thanks michael!
/lgtm

[ack-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: a-hilaly, michaelhtm

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