adds http-checksum trait support

[skotambkar]

Status: Needs review

Description of changes:
[x] Adds httpChecksum trait to smithy.
[x] Adds test cases and docs

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[mtdowling]

Did we consider making these a little more explicit, customizable, and future proof by using a map similar to:

@httpChecksum(
    request: {
        sha256: {
            header: "x-checksum-sha256"
        },
        crc32: {
            header: "x-checksum-crc32"
        }
    }
)

Here, request and response are maps where each key is the algorithm, and each value is a union of possible locations (i.e., it can be set to header or trailer, and we could expand to query in the future for requests if needed). The value of the union is the binding location name (i.e., header name or trailing header name).

This provides more flexibility than the current proposal because each algorithm can use whatever header or query string parameter is necessary. The header/query binding is explicit and does not need prefixes. The names can now support anything, including Content-MD5 if we ever wanted to support it (we probably don't but just an example).

Implementations don't need to perform any prefix logic to determine the header name either.

[skotambkar]

I like the idea, have few concerns. Let's sync offline on this.

[skotambkar]

Replaced by #843