SigningRequest should preserve existing raw query when applying signed requests

[QuakeWang]

Description

reqsign-core::SigningRequest currently parses the request URI query with form_urlencoded::parse in SigningRequest::build, stores decoded query pairs, and rebuilds the URI query from those pairs in SigningRequest::apply.

This can change the wire representation of query parameters that already existed before signing.

For example, an existing query like:

versionId=a%2Bb%3Dc%2525%26e

can be rebuilt into a raw query like:

versionId=a+b=c%25&e

The rebuilt URI no longer preserves the original encoded value. In particular, the encoded & inside the value can become a query separator, and + can also become ambiguous for form-style query parsers.

This matters for downstream users such as OpenDAL. In apache/opendal#7888, COS requests need to keep an encoded versionId query parameter after signing.

Fixing this in OpenDAL requires service-local query restoration logic, but the root issue appears to be that reqsign-core does not preserve the raw query representation across SigningRequest::build and SigningRequest::apply.

Review context: apache/opendal#7888 (review)

Expected Behavior

Signing should not rewrite query parameters that already existed in the request URI.

Signers may still parse and canonicalize query parameters for signature calculation. Query-signing implementations may also append authentication query parameters. However, existing raw query parameters should keep their original wire representation unless a signer explicitly updates them.

Scope

This issue is about reqsign-core::SigningRequest, not a COS-specific workaround in OpenDAL.

At least reqsign-tencent-cos is affected because it builds on top of SigningRequest. Other signers using SigningRequest may need to be checked as well.