dtls: dissect tls extensions

This adds dissection for more TLS extensions. The code was mostly
copied from the SSL dissector.

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>