PSS: Update how commands access backends, so both backend and state_store configuration can be used

[SarahFrench]

Context / How code before this PR worked

Terraform command implementations access operations backends through the (Meta).Backend method. This operations backend may be used for accessing state and/or for running operations.

When using the (Meta).Backend method, calling code may include parsed backend configuration in the BackendOpts argument passed into the Backend method:

// Get the backend config from the root module
backendConfig, configDiags := c.loadBackendConfig(".")
// handle configDiags errors etc

// Pass the config into the Backend method
be, beDiags = c.Backend(&BackendOpts{
	BackendConfig: backendConfig,
	ViewType:      viewType,
})

Or, the calling code could pass a nil value into the Backend method and instead rely on downstream logic to parse any backend blocks in the configuration (this happens if the BackendOpts.BackendConfig value is missing).

// Load the backend
b, backendDiags := c.Backend(nil)

Changes in this PR

This PR adds two new methods:

• (m *Meta) getStateStoreProviderFactory
• (m *Meta) prepareBackend

*(m Meta) prepareBackend - see commit 2ecbed9. This method makes it easier to access an instance of an operations backend. This method parses any backend or state_store configuration in the root module, provides that input to the (Meta).Backend method, and returns the operations backend from there.

*(m Meta) getStateStoreProviderFactory - see commit 60d5333. This method makes it easy to quickly pull the factory needed for a state store from the available providers, or return the correct errors if data or a provider is missing.

Overall, this PR makes Terraform commands compatible with state stored via state_store. The exceptions are commands like terraform cloud * which expect a specific type of state storage to be present in the configuration when the user executes the command.

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]

This currently doesn't have any test coverage

[SarahFrench]

I have a (blocked) PR in draft that adds the ability to pass locks into the methods used for initialising an operations backend that uses state_store. While that work is in progress there are some parts of this code that aren't possible yet but I think it's reasonable to get this completed version into main, to help make other subsequent PRs smaller.