Improve RSA

[ApolloZhu]

https://github.com/ApolloZhu/BilibiliKit/blob/f2300b9413a31f2b1a114804aa893deaf669da31/Sources/BKSec.swift#L16-L42

As demonstrated in the documentation, we should

• [x] check if it is supported
• [x] some other checks in the documentation
• [x] take retained value instead of using String(describing:)