net/tls: Move TLS protocol elements to a separate header

Kernel TLS consumers will need definitions of various parts of the
TLS protocol, but often do not need the function declarations and
other infrastructure provided in <net/tls.h>.

Break out existing standardized protocol elements into a separate
header, and make room for a few more elements in subsequent patches.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://lore.kernel.org/r/169047931374.5241.7713175865185969309.stgit@oracle-102.nfsv4bat.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>