x/crypto/ssh: Support sha-2 family hash algorithms for key exchange (RFC 8268)

[breml]

Feature Request: Add support for sha-2 family hash algorithms for key exchange (RFC 8268)

Due to concerns with SHA-1 and with MODP groups with less than 2048 bits, RFC 8268 suggests to add 5 new key exchange algorithms:

• diffie-hellman-group14-sha256
• diffie-hellman-group15-sha512
• diffie-hellman-group16-sha512
• diffie-hellman-group17-sha512
• diffie-hellman-group18-sha512

Currently, non of these new key exchange algorithms is present in x/crypto/ssh. For diffie-hellman-group14-sha256 the RFC states it SHOULD be supported to smooth the transition to newer group sizes.

[hanwen]

But why? There are a number of more modern kexes based on elliptic curves, which are faster to process and (AFAIK) as secure.

[bcmills]

CC @hanwen @FiloSottile