Fix AWS SigV4 header whitespace canonicalization

[ryanfowler]

Summary

• Canonicalize each AWS SigV4 header value before signing
• Collapse internal whitespace runs to a single space and join multi-value headers with commas
• Avoid extra allocations for already-canonical single header values
• Add regression coverage for multi-value and internal-whitespace header cases

Testing

• go test -v ./internal/aws
• go test -v ./...