Updated RSA Public API

[btoms20]

Fixes #980

Checklist:

• Correct file headers (see CONTRIBUTING.md).
• Formatted with SwiftFormat.
• Tests passed

Changes proposed in this pull request:

• Changed the following initializer and methods on the RSA class from internal to public

init(rawRepresentation: Data) throws

func externalRepresentation() throws -> Data

func publicKeyExternalRepresentation() throws -> Data

@nathanfallet - I went ahead and left the DERCodable protocol internal and just updated the RSA methods to be public in order to only expose the important / necessary methods.

[nathanfallet]

@btoms20 Why did you chose to make only some methods public?

[btoms20]

• The externalRepresentation() method is just a wrapper around the publicKeyDER() & privateKeyDER() methods.
• Similarly, the init(rawRepresentation:) is a wrapper around init(privateDER:) & init(publicDER:).
• At this point the public facing api is similar to that of Apple's Security framework.