PSS : Add inmem state storage implementation to the builtin terraform provider, update grpcwrap package, use PSS implementation in E2E test

[SarahFrench]

This PR copies a bunch of code from the existing inmem backend and uses it to create an inmem version of a pluggable state store in the builtin terraform provider.

This will only be accessible if a user has TF_ACC set in their environment, which is a standard ENV for enabling test-specific behaviour or actions. This won't fully block a user from accessing the state store, but it provides more friction than the current inmem backend, which is just not documented.

I'm sharing this work for feedback before adding it to terraform-provider-simple6, for E2E testing

Target Release

N/A

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

[SarahFrench]

For context, the inmem backend uses these global vars:

terraform/internal/backend/remote-state/inmem/backend.go

Lines 24 to 30 in efb2a1c

	// we keep the states and locks in package-level variables, so that they can be
	// accessed from multiple instances of the backend. This better emulates
	// backend instances accessing a single remote data store.
	var (
	states stateMap
	locks lockMap
	)

Like the code comment says, multiple backends can attempt to access the same state by using the same providers.Interface instance to create them.

[SarahFrench]

Something to bear in mind is that E2E tests also run in the context of the build workflow. I think it'd best to avoid enabling experiments to leak to that workflow, and instead have experimental tests skipped when running in that context.