Feature: add support for aws-chunked content encoding (#1501)

* add: support for unsigned aws-chunked content encoding

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

* refactor: API of AwsChunkedBodyOptions
update: AwsChunkedBody to note we only support single chunks
remove: unnecessary `Option`s
remove: overly smart total_length_of_trailers_in_bytes in trailers_as_aws_chunked_bytes
update: use "where"-style declaration for `impl<Inner> Body for AwsChunkedBody<Inner>`
add: helpful data to trace logging
add: trailer len double check in AwsChunkedBody::poll_data
add: test for trailer len double check
add: assert to size_hint
fix: incorrect body emitted when body is empty
add: test for empty encoded body

* Update CHANGELOG.next.toml

Co-authored-by: John DiSanti <jdisanti@amazon.com>

* update: CHANGELOG.next.toml message

* refactor: aws-chunked trailer encoding and size calculation
update: when aws-chunked formatting a `HeaderMap`, header names with multiple values will be written out one value per line
remove: unnecessary AwsChunkedBodyOptions::stream_length method
add: trace fields
refactor: make inserting the final body CRLF more explicit
add: test to ensure trailer encoding and trailer len calculation stay in sync

* udpate: emit error instead of panicking for incorrect trailer length

* add: test ensuring trailer serialization and length calculation stay in sync for empty HeaderMap

* update: avoid repeated allocations in trailers_as_aws_chunked_bytes
fix: don't convert to str before getting len of HeaderValue

* add: aws-chunked body test for inner bodies that emit intermittent chunks
add: double check that stream_length used to create an `AwsChunkedBody` is correct.

Co-authored-by: John DiSanti <jdisanti@amazon.com>