[11u] RFR: 8206925: Support the certificate_authorities extension

[Doerr, Martin]

Hi,

JDK-8206925 was backported to 11.0.10-oracle, but it's still missing in the Open Source version.
I'd like to backport it for parity.
It does apply cleanly, but I had to modify it, because the following change is not in 11u:
https://bugs.openjdk.java.net/browse/JDK-8215712

Bug:
https://bugs.openjdk.java.net/browse/JDK-8206925

Original change:
https://hg.openjdk.java.net/jdk/jdk/rev/827bac238aa0

11u backport:
http://cr.openjdk.java.net/~mdoerr/8206925_ca_ext_11u/webrev.00/

Manual change to make it work without JDK-8215712 (SSLStringizer and derived classes don't take a HandshakeContext in 11u):
http://cr.openjdk.java.net/~mdoerr/8206925_ca_ext_11u/8206925_ca_ext_diff.txt

Please review.

Best regards,
Martin