Rawtransactions: joinpsbts - Implement REST API endpoint for Bitcoin Core method

Implement a REST API endpoint for the `Rawtransactions`: `joinpsbts` method of the Rawtransactions category in Bitcoin Core.

No file or test is named. Start by inspecting existing REST endpoints for the Rawtransactions category and their Bitcoin Core JSON-RPC mappings, then locate the endpoint registration and request handling pattern for a comparable method. Done means the REST API exposes Rawtransactions.joinpsbts and returns its Bitcoin Core response.

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