add: callback trait proposed in rfc#0012 (#1329)

* add: callback trait proposed in rfc#0012
add: callback setting API to ByteStream
add: callback setting API to SdkBody
add: callback tests for streaming and buffered data
add: private callback fns for calculating checksums
udpate: pub methods on private struct `Inner` to be private
update: changelog
remove: Sync bound from SdkBody test "sdk_body_is_send()"
add: fn to merge `HeaderMap`s by appending
add: tests for new functionality

* add: Sync bound to callbacks
refactor: poll_trailers method
update: split out checksum callbacks into their own module
formatting: use throwaway let binding instead of allow unused variable

* add: license file to new crate
add: readme file to new crate

* add: missing anchor to Cargo.toml

* rename: aws-smithy-checksum-callbacks to aws-smithy-checksums
add: two more append_merge_header_maps tests
update: make append_merge_header_maps pub(crate)
remove: check for non-existent log line

* remove: doc test for private fn

* update: checksum callback struct names