Add file and multipart request body support (#43, #84) - #168

Summary

• Add request_body_file and request_multipart across terracurl_request resources, data sources, actions, and ephemeral resources (including read/destroy/renew/close variants where applicable).
• Introduce shared payload resolution (resolveRequestPayload) with file read, multipart builder, Content-Type handling, and per-operation mutual-exclusion validators.
• Extend destroy templating to substitute {response.*} in multipart part value fields at delete time.
• Add guide, examples, generated schema docs, and CHANGELOG entry for 2.10.0. Closes Feature Request: Specify Files For Request Body #43 and [Feature Request] Support for multipart/form-data Format #84.

Test plan

• go test ./internal/provider/ (unit + acceptance)
• Resource PUT with request_body_file
• Resource POST with request_multipart (text + file part)
• Data source read with request_body_file
• Action multipart payload wiring
• Ephemeral renew/close with snapshotted renew_request_body_file / close_request_body_file

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