PSS: Add savedStateStore method to Meta

[SarahFrench]

Description

This PR adds a new savedStateStore method that's similar to the existing savedBackend method on Meta.

The savedBackend method on Meta returns a backend that's configured using the backend state file's contents and isn't influenced by the current config. This method is used during state migration events in init commands, where Terraform needs to have an instance of the old backend, configured the old way, in order to access state to migrate to the new backend's location.

Note: There is a variant called backendFromState that does a similar job but isn't considered in this PR. This variant is specific to users running init commands with -backend=false as a flag.

This PR adds a new savedStateStore method that's similar to the existing savedBackend method on Meta. The method uses only backend state file data to configure and return a backend.Backend instance made using a provider that implements pluggable state storage.

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]

From discussion - will need to remove marks for provider config, but state store config currently is unable to include 'sensitive' etc values so marks currently not possible.

Remove marks and add tests to confirm

I've attempted to write a test that asserts marks are removed, but marks don't appear to be present at the time they're being removed. I assumed that this code that uses the schema (which contains Sensitive attributes) would result in a cty.Value that contains marks. Is there something wrong with my assumptions, or wrong with the code itself?

[SarahFrench]

I sought some guidance about the question of whether marks are relevant here at all.

From what I learned code in Terraform should only adds marks to config values that are used in the presentation layer, i.e. things that would be shown in the terminal output. They would be removed again when persisting to state, etc.

I briefly implemented adding the marks (483f57a) but as there are no diagnostics that would log sensitive config values to the terminal those marks are not used anywhere. So I've updated the PR to no longer add the marks. In the code that uses the RPCs related to configuring providers and state stores we no longer have any code for removing marks as we now know that we're not adding any to those values, regardless of what the matching schema says about sensitivity etc. I've put a comment describing this to the code, as others might make our mistake of blindly assuming marks need to be removed before calling any RPC.

[SarahFrench]

Thanks!