dtls: extract check for client auth
dtls: extract check for client auth

Add functions is_ecdsa_client_auth_supported() which checks if the
application code requests client auth.

Signed-off-by: Hauke Mehrtens <435ddd46dc66c007a1fa20144c823d37e0d23436@hauke-m.de>

spark/tinydtls,nmacs/tinydtls,thecodemaiden/tinydtls-fork,OlegHahm/tinydtls,yunus/TinyDTLS-WebID,fvdnabee/tinydtls,fvdnabee/tinydtls,OlegHahm/tinydtls,jvermillard/tinydtls,nmacs/tinydtls,yunus/TinyDTLS-WebID,thecodemaiden/tinydtls-fork,yunus/TinyDTLS-WebID,jvermillard/tinydtls,nmacs/tinydtls,jvermillard/tinydtls,thecodemaiden/tinydtls-fork,nmacs/tinydtls,fvdnabee/tinydtls,spark/tinydtls,OlegHahm/tinydtls,spark/tinydtls