* 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
