PSS: Add parsing of `.tfmigrate.hcl` files to define state migration operations (#38526)

* feat: Parsing a configuration directory can include .tfmigrate.hcl files. These files include `state_store_provider` blocks and `from` blocks that contains either a single `backend` or `state_store` block.

Validation will enforce that despite multiple .tfmigrate.hcl files being parsed you can only have one state_store_provider or from block, and nested backend/state_store blocks are mutually exclusive. Use of state_store_provider is only valid when state_store is in use, and the single provider described in both blocks must be in agreement. Also, a directory's .tfmigrate.hcl files cannot be empty once combined.