* Drop DC test data for draft23

* DC: Change public key type

There was a last minute change to spec that changes the public key type
from a byte string of length at most 2^16 to a byte string of length at
most 2^24.

* includes AD in authentication check of TLS records

As per 5.2 of TLS 1.3 draft-28, the additional data is record header.

* tests: Update tests in order to support draft-28

* Interoperability: Updates NSS and BoringSSL versions to the
  one supporting draft-28
* Bogo: Updates revision number to use tests for draft-28
* FIX: makefile was using test-compat target instead of
  test-interop

* DC test: constify

* Use binary interface to encode in big-endian

The test in subcerts_test.go only passes if maxVersion ==
VersionTLS13Draft23. This is because DCs are cryptographically bound to
the protocol version on the wire. To work around this as we move towards
the RFC, this PR adds test data for VersionTLS13Draft28 and VersionTLS13
and uses maxVersion to pick which data to load.  # Please enter the
commit message for your changes. Lines starting

Drops support for delegated credentials with TLS 1.2 and adds the
protocol version and signature algorithm to the credential structure.

What's left are the minimal API changes required to use the delegated
credential extension in the TLS handshake.

This removes dependency on NewDelegatedCredential from tris.

This complies with the standard, currently an Internet draft:
https://tlswg.github.io/tls-subcerts/draft-ietf-tls-subcerts.html

It also adds a minimal interface for generating new delegated
credentials.