[WIP] RSABackend for CC (Security.Framework)

This is a WIP RSABackend that is being pushed so everyone can see current progress. It currently fails due to testing issues enumerated in #1074. #1082 and #1083 are also relevant to eventually landing this.

Ultimately this PR will be split up to be landed. It should also land after the `*Numbers` refactor so the OpenSSL-based workarounds currently present in some sections will be unnecessary.

(refs #1078)