Add RFC for finding new home for credential types (#2082)

* Add RFC: new home for cred cache and builder api consistency

* Update design/src/rfcs/rfc0027_new_home_for_cred_cache_and_builder_api_consistency.md

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

* Update design/src/rfcs/rfc0027_new_home_for_cred_cache_and_builder_api_consistency.md

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

* Update rfc0027_new_home_for_cred_cache_and_builder_api_consistency.md

* Update rfc0027_new_home_for_cred_cache_and_builder_api_consistency.md

* Update rfc0027_new_home_for_cred_cache_and_builder_api_consistency.md

* Update design/src/SUMMARY.md

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

* Update design/src/rfcs/rfc0027_new_home_for_cred_cache_and_builder_api_consistency.md

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

* Update design/src/rfcs/overview.md

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

* Update the filename of RFC

* Update rfc0029_new_home_for_cred_types.md

* Update status of RFC 28

This commit addresses https://github.com/awslabs/smithy-rs/pull/2082#discussion_r1054922093

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Co-authored-by: Yuki Saito <awsaito@amazon.com>
Co-authored-by: John DiSanti <jdisanti@amazon.com>
Co-authored-by: Zelda Hessler <zhessler@amazon.com>