* Implement `ForPlan` method on `StateStoreConfigState`, add `Planner` interface
* Rename `ForPlan` method to `Plan`
* Allow plan files to contain information about state stores
* Add code needed for representing a state store in the Plan struct, which is used when handling plan files
* Add ability to read/write either a backend or state store's data in a plan file. Add some test coverage.
* Update plan's `ProviderAddrs` method to include the provider used for PSS, if present
* Split interfaces
* Apply feedback from code review
* Refactor `SetVersion` to use appropriate constructor
* Split `ProviderAddrs` method test into two
* Fix method call after rename
* Fix test
* Remove change to `(p *Plan) ProviderAddrs()`
We may re-add this when we implement PSS for use during apply commands with plan files
* Remove changes to test, now that the plan doesn't report the provider used for PSS anymore.
* Split code for backend state file vs backend state
* Rename BackendState to BackendConfigState
* Spelling error
* Add `StateStorageConfigState` struct as new implementation of new `ConfigState[T any]` interface.
* Split tests for backend state file vs backend config state structs
* Rename StateStorageConfigState to StateStoreConfigState
* Clarify test name, add comments
* Add tests for StateStoreConfigState methods
* Add test showing state_store in JSON is parsed correctly
* Add detection of malformed backend state files that contain both backend and state_store fields
* Add validation that stops a backend state file being written if it will contain state for both backend and state_store blocks
* Rename `state_storage` to `state_store`
* Rename `state_storage` to `state_store` in filenames
* Move`ConfigState` to its own file
* Fix test name, remove whitespace
* Update `StateStoreConfigState` comment using review suggestion
* Update error message to no longer allude to the environment TF is being run in
* Update the state_store state to use `version.Version` and an adapted version of `tfaddr.Provider` for marshalling version and source data
* Update test helper so it doesn't accidentally supply validation in tests
* Add protection against saving an empty backend state file
* Remove direct testing of (s *Source) MarshalText() and UnmarshalText() methods
* Add Validate method to StateStoreConfigState, use in backend state encoding logic
* Refactor to use new features in registry dependency
