Fix AWS SigV4 compatibility with Supabase Storage S3

[meeq]

The AWS SigV4 documentation states:

The CanonicalHeaders list must include the following:

• HTTP host header.
• If the Content-MD5 header is present in the request, you must add it to the CanonicalHeaders list.
• Any x-amz-* headers that you plan to include in your request must also be added. For example, if you are using temporary security credentials, you need to include x-amz-security-token in your request. You must add this header in the list of CanonicalHeaders.

Supabase Storage explicitly forbids certain headers from being included in the signature, so this patch pares down the canonical headers to a more broadly-supported subset.

For reference, botocore (the underlying library that powers the canonical aws-cli tool) only signs "interesting" headers, and omits the rest, so this change should not break compatibility.

Related to wojtekmach/req_s3#22