Add RFC: Providing fallback credentials on timeout (#2218)

* Add RFC: providing fallback credentials on timeout

* Update design/src/rfcs/rfc0031_providing_fallback_credentials_on_timeout.md

Co-authored-by: Zelda Hessler <zhessler@amazon.com>

* Update design/src/rfcs/rfc0031_providing_fallback_credentials_on_timeout.md

Co-authored-by: Zelda Hessler <zhessler@amazon.com>

* Update design/src/rfcs/rfc0031_providing_fallback_credentials_on_timeout.md

Co-authored-by: Zelda Hessler <zhessler@amazon.com>

* Update design/src/rfcs/rfc0031_providing_fallback_credentials_on_timeout.md

Co-authored-by: Zelda Hessler <zhessler@amazon.com>

* Update design/src/rfcs/rfc0031_providing_fallback_credentials_on_timeout.md

Co-authored-by: Zelda Hessler <zhessler@amazon.com>

* Update design/src/rfcs/rfc0031_providing_fallback_credentials_on_timeout.md

Co-authored-by: Zelda Hessler <zhessler@amazon.com>

* Update design/src/rfcs/rfc0031_providing_fallback_credentials_on_timeout.md

Co-authored-by: Zelda Hessler <zhessler@amazon.com>

* Update design/src/rfcs/rfc0031_providing_fallback_credentials_on_timeout.md

Co-authored-by: Zelda Hessler <zhessler@amazon.com>

* Update design/src/rfcs/rfc0031_providing_fallback_credentials_on_timeout.md

Co-authored-by: Zelda Hessler <zhessler@amazon.com>

* Update design/src/rfcs/rfc0031_providing_fallback_credentials_on_timeout.md

Co-authored-by: Zelda Hessler <zhessler@amazon.com>

* Update design/src/rfcs/rfc0031_providing_fallback_credentials_on_timeout.md

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

* Incorporate review feedback into RFC

This commit addresses the review feedback:
https://github.com/awslabs/smithy-rs/pull/2218#discussion_r1072657848
https://github.com/awslabs/smithy-rs/pull/2218#discussion_r1072685567
https://github.com/awslabs/smithy-rs/pull/2218#discussion_r1072693150



In addition, we have renamed the proposed method `on_timeout` to
`fallback_on_interrupt` to make it more descriptive.

* Update rfc0031_providing_fallback_credentials_on_timeout.md

* Update RFC

This commit updates RFC and leaves to discussion how `fallback_on_interrupt`
should be implemented, i.e., either as a synchronous primitive or an
asynchronous primitive.

* Update rfc0031_providing_fallback_credentials_on_timeout.md

* Update rfc0031_providing_fallback_credentials_on_timeout.md

* Update rfc0031_providing_fallback_credentials_on_timeout.md

---------

Co-authored-by: Yuki Saito <awsaito@amazon.com>
Co-authored-by: Zelda Hessler <zhessler@amazon.com>
Co-authored-by: John DiSanti <jdisanti@amazon.com>
Co-authored-by: Luca Palmieri <20745048+LukeMathWalker@users.noreply.github.com>