Feature: checksum related body-wrappers (#1502)

* add: checksum related body-wrappers
add: tests for checksum related body-wrappers

* add: aws-smithy-checksums to CrateSet.key

* add: missing base64::encoded_length fn

* update: use the checksum digest trait's output_size method to get checksum size
add: proptest for base64 encoded length
remove: old base64 encoded length test
remove: incorrect leftover comments and comment quotes

* remove: unnecessary pin

* remove: use of clone during checksum finalization
refactor: checksum fns are no longer fallible
refactor: HttpChecksum is now responsible for reporting size correctly
fix: don't consider trailers as part of ChecksumBody size hint

* remove: Taken error state
update: prefer if-let to match
fix: incorrect is_end_stream impl
refactor: use default trait impl for HttpChecksum::header_value

* fix: clippy lint

* add: header value tests

* undo: accidental retry changes

* add: http-checksum-related errors

* refactor: break up and modularize body wrappers
update: docs
update: ChecksumValidatedBody::new now takes a Box<dyn HttpChecksum>

* fix: broken test

* refactor: rename checksum body structs to be differentiated only by their module