Initial APIs for making serialization encryption configurable

[alex]

closes #7441

[reaperhulk]

enc_type = KeySerializationEncryptionBuilder().kdf_rounds(32).build(b"password") is the usage model then... And a more complex thing might look like:

KeySerializationEncryptionBuilder().mac_algorithm(SOME_ENUM).
    cipher(SOME_OTHER_ENUM).
    mac_rounds(100).
    kdf_rounds(300).
    build(b"password")

Do we have separate builders for types (e.g. PKCS12SerializationEncryptionBuilder)? Is SSH a distinct type? I think this approach is a reasonable one though.

[alex]

I was going to just use the same type for each, and then have things check if the method was valid for that _format.