Closes#1648Fixes#1980Fixes#1981
This PR implements browser-based POST object uploads for S3-compatible form uploads. It adds support for handling `multipart/form-data` object uploads submitted from browsers, including streaming multipart parsing so file content is not buffered in memory, POST policy decoding and evaluation, SigV4-based form authorization, and integration with the existing `PutObject` backend flow. The implementation covers the full browser POST upload path, including validation of required form fields, credential scope and request date checks, signature verification, metadata extraction from `x-amz-meta-*` fields, checksum field parsing, object tagging conversion from XML into the query-string format expected by `PutObject`, and browser-compatible success handling through `success_action_status` and `success_action_redirect`. It also wires the new flow into the router and metrics layer and adds POST-specific error handling and debug logging across policy parsing, multipart parsing, and POST authorization. AWS S3 also accepts the `redirect` form field alongside `success_action_redirect`, but since AWS has marked `redirect` as deprecated and is planning to remove it, this gateway intentionally does not support it.
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