Add AWS auth and error foundations (#108)

* Add AWS auth and error foundations

- Adds dependency-free SigV4 parsing, known-key auth modes, explicit missing and unknown auth states, and gateway identity context.

- Adds AWS XML, S3 REST XML, and JSON error serializers for SDK-compatible service failures.

- Covers header, presigned, relaxed, known-key, strict, gateway, and error serialization behavior in Go tests.

* Harden AWS auth resolution

* Fix AWS auth and error defaults