Add ability to produce PKCS#1 signatures using RSA keys of 1024-2047 bits (inclusive)

[briansmith]

Provide a way for users of RSA private keys to opt into support for keys with public modulus less than 2048 bits and at least 1024 bits.

ring already has a way of opting into verifying signatures with small keys, but it doesn't have a way to use such keys to produce new signatures. However, some users have a need to support existing keys that have a smaller key. Although this is over-simplifying things, the security risk of ultimately on the side of the verifier, which probably shouldn't accept such small keys.