fix: Ignore changes to snapshot_identifier

[robinbowes]

Description

Ignore changes to snapshot_identifier. Fixes #270

Motivation and Context

snapshot_identifier is only used when creating the DB instance. However, if the parameter is changed post-creation it causes the DB instance to be deleted and re-created.

This is the solution recommended by Hashicorp: https://discuss.hashicorp.com/t/rds-restored-from-snapshot-is-destroyed-on-next-terraform-apply/3693/4

Breaking Changes

This change should not break anything, unless a user is relying on changing this parameter to trigger the creation of a DB instance.

How Has This Been Tested?

We are using this change in our private fork of this module.

[robinbowes]

Close/re-open to re-trigger tests

[okgolove]

I'd suggest having two aws_db_instance resources. One with ignore_changes, one without.
In some cases I want to trigger recreating db instance based on snapshot identifier.

[bryantbiggs]

hi @robinbowes - thanks for the PR. this is always something very tricky/difficult to work with in modules (lifecycle hooks). I think the safest approach is as @okgolove stated but that adds a significant amount of overhead and complexity for little gain - I'm not sure what we will be able to do here (hoping Hashicorp provide a variabilzed version of lifecycle hooks soon 🤞🏼)

[bryantbiggs]

closing this in favor of what is provided by the latest release of the upstream AWS provider hashicorp/terraform-provider-aws#18013

[github-actions]

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