RFC: How Cargo "features" should be used in the SDK and runtime crates (#1067)

* in-progress draft of features RFC

* add: info on default feature functionality change

* Update design/src/rfcs/rfc0009_using_features_responsibly.md

Co-authored-by: Russell Cohen <rcoh@amazon.com>

* rename: RFC filename
add: rfc to summary page
remove: TODO section

* update: design summary ToC
remove: default feature example

* fix: RFC numbers in SUMMARY ToC

* fix: bad link

Co-authored-by: Russell Cohen <rcoh@amazon.com>