x509af: dissect subjectPublicKey for RSA

x509af: dissect subjectPublicKey for RSA

The subjectPublicKey field of a Certificate (TBSCertificate) is defined
as type BIT STRING. The actual contents depend on the Algorithm
Identifier which is preceding the subjectPublicKey field.

This patch adds support for dissection of the public key for RSA
public keys which show up below the subjectPublicKey tree:

    subjectPublicKeyInfo
        algorithm (rsaEncryption)
            Algorithm Id: 1.2.840.113549.1.1.1 (rsaEncryption)
        subjectPublicKey: 3082010a0282010100b7c769e2d0eacaeb929fc08238a9ff...
            modulus : 0x00b7c769e2d0eacaeb929fc08238a9ffc59cab39c28a2e26...
            publicExponent: 65537

Change-Id: Ib92645433b0a0078a947ff0ac26c5e6a64877b93
Reviewed-on: https://code.wireshark.org/review/10967
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>