dtls: parse supported_signature_algorithms in CertificateRequest

DTLS 1.2 adds a supported_signature_algorithms attribute to the CertificateRequest which was not parsed in DTLS.

This code was copied from the ssl code.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
tmp: add basic support for TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 and raw public keys

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>