Add serverside encryption headers as request params

[nadeesha]

Brief description of the changes

This allows us to specify custom encryption keys via aws:kms. It solves the problem that #1803 is trying to solve. With this, you can just do the following to get KMS keys working

      request: {
        endpoint: "...",
        accessKey: "...",
        params: {
          "x-amz-server-side-encryption": "aws:kms",
          "x-amz-server-side-encryption-aws-kms-key-id": "JE9Yya8Rd6jcbZbb8ymFoU6prbpyyZDc"
        }
      },

What browsers and operating systems have you tested these changes on?

Chrome. But this is unlikely to break any behaviour that is already not broken by custom params.

Have you written unit tests? If not, explain why.

The current custom params implementation does not contain unit tests.

[rnicholus]

This seems simple enough. I’ll try to merge it in soon.