Add support for Plan Checks with plancheck.ExpectResourceAction, plancheck.ExpectEmptyPlan, and plancheck.ExpectNonEmptyPlan

[austinvalle]

• closes Core FR: Check that only an 'update' occurred in a test #61
• implementation of PlanChecks RFC TF-633

This PR introduces generic plan check functionality that can be run at different points in a test step (pre-apply, post-apply, etc). These checks can inspect the plan and return an error to fail the test.

The plancheck.PlanCheck interface can be implemented by provider developers, however, this PR contains a new package plancheck that contains implementations for common use cases:

• plancheck.ExpectResourceAction: For #61 - Allow asserting that a resource will undergo a specific action type, such as Create, Update, NoOp, Replace, etc.
• plancheck.ExpectEmptyPlan: Allows asserting that a plan is empty
• plancheck.ExpectNonEmptyPlan: Allows asserting that a plan is not empty

Work left for this PR

• Continue investigating if we need ResourceActionType or if we can re-use tfjson.Action
• Add content for the new documentation page in website/docs/plugin/testing/acceptance-testing/plan-checks.mdx
• Add changelogs for feature enhancement and new package/subsystem

[bflad]

The RFC overall looks good to me, so I'll start code commenting for tactical feedback. 👍

[bendbennett]

Looks great! Nice work.

[bflad]

Looks good to me 🚀

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