feat(aws): support configurable canonical URI encoding

[unconsolable]

Summary

• Add PercentEncodingMode::{Single, Double} and with_percent_encoding_mode(...) to the SigV4 and SigV4a signers, covering both header signing and presigning.
• Keep Single as the default, preserving existing behavior. Double re-encodes the original wire path without decoding it first, while preserving literal slash separators.
• Preserve the outgoing request path and existing query bytes; presigning only appends authentication parameters.
• Add AWS SDK signature comparisons, edge-case regression tests, and API/README documentation.

Validation

• Passed formatting, strict workspace Clippy, workspace tests, all-feature doctests, and focused AWS tests.
• Passed WASM checks for the affected AWS crates and the facade without default-context.

Compatibility

Single retains Reqsign’s existing per-segment decode/re-encode behavior rather than changing it to the AWS SDK’s raw-path behavior.

closes #911