Add X-Amz-Server-Side-Encryption-Context header to required signed headers allowlist

[fenak]

Summary

Adds the X-Amz-Server-Side-Encryption-Context header to required signed headers allowlist

This PR is to keep parity with aws/aws-sdk-go#4949

Problem description

When presigning a PutObjectRequest which includes SSEKMSEncryptionContext in its s3.PutObjectInput parameter, the X-Amz-Server-Side-Encryption-Context gets added to the URL as a query parameter, not as a signed header.

When using the presigned URL to put an object in a bucket, you can't send the encryption context as a header as it wasn't on the list of signed headers, and the parameter sent via URL doesn't work (the object is uploaded and encrypted without adding the encryption context to it).

As mentioned in the Specifying server-side encryption with AWS KMS (SSE-KMS) documentation, at section "Using the REST API":

When you create an object—that is, when you upload a new object or copy an existing object—you can specify the use of server-side encryption with AWS KMS keys (SSE-KMS) to encrypt your data. To do this, add the x-amz-server-side-encryption header to the request. Set the value of the header to the encryption algorithm aws:kms. Amazon S3 confirms that your object is stored using SSE-KMS by returning the response header x-amz-server-side-encryption.

If you specify the x-amz-server-side-encryption header with a value of aws:kms, you can also use the following request headers:

x-amz-server-side-encryption-aws-kms-key-id

x-amz-server-side-encryption-context

x-amz-server-side-encryption-bucket-key-enabled

I could verify that using my fork the request is the encryption context sent via headers, and then it works as expected.

This fix seems to be aligned to the behaviour mentioned in the aws/aws-sdk-js-v3 repo:

• https://github.com/aws/aws-sdk-js-v3/blob/8c2e326138dccd9d98052160e36492d06696bad8/packages/s3-request-presigner/src/presigner.ts#L32-L39
• https://github.com/aws/aws-sdk-js-v3/blob/8c2e326138dccd9d98052160e36492d06696bad8/packages/s3-request-presigner/README.md?plain=1#L37-L39