RFC: Callbacks for `ByteStream` and `SdkBody` (#1307)

* add: initial draft of bytestream callback RFC

* add: builder-style method

* update: remove callback insert method in favor of builder method
update: Arc callbacks instead of boxing them
remove: most bounds from ByteStreamReadCallback trait
add: PartialEq/Eq impl for Inner that disregards callback list

* update: RFC with SdkBody impl
update: RFC with checksum callback example

* update: respond to PR comments

* update: callback impls

* remove: Sync bounds from callback traits

* update: consolidate and simplify callback API

* fix: leftover terminology