remove: default features from runtime crates (#935)

* remove: default features from runtime crates
update: tests and examples broken by default feature change
update: code generation broken by default feature change
fix: follow lint suggestion to lowercase variable names

* update: CHANGELOG.next.toml

* fix: add missing feature
fix: typo
formatting: sort imports

* update: make SDK changelog less worrying for SDK users not depending on a runtime crate
update: undo default feature change for SDK
fix: tests broken by default feature change
fix: examples broken by default feature change

* remove: empty defaults
add: doc w/ feature grid to aws-smithy-client
add: doc w/ feature grid to aws-smithy-http

* add: upgrade helper table to changelog
undo: bad formatting
formatting: add backticks to feature names in doc table

* remove: bytestream-util feature usage where possible
formatting: update CHANGELOG.next.toml

* remove: the client feature from generated SDKs
rename: feature "bytestream-util" to "rt-tokio"
update: mark "tower" dep as non-optional in SDKs
docs: update aws-smithy-http docs and README.md

* Update CHANGELOG.next.toml

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

* Update rust-runtime/aws-smithy-http/README.md

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

* revert: aws-sigv4 changes
update: CHANGELOG.next.toml
remove: rt-tokio dep feature for aws-smithy-async usage in aws-types

* fix: don't gate entire byte_stream module on rt-tokio feature
fix: typos

* remove: rt-tokio feature from SDKs generated with EventStreamSymbolProvider.kt
fix: incorrect doc comment

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