Add support for EncryptedClientHello

[alexw91]

Problem:

s2n-tls does not support this draft RFC for Encrypted ClientHello (ECH), which is nearing final standardization. This RFC feature was previously called Encrypted Server Name Indicator (ESNI), but has since been updated to also support encrypting the ClientHello.

• https://datatracker.ietf.org/doc/html/draft-ietf-tls-esni

Solution:

s2n-tls should add support for this RFC.

Requirements / Acceptance Criteria:

TLS Clients connecting to s2n-tls server can successfully follow this draft RFC to encrypt their ClientHello.

• RFC links: https://datatracker.ietf.org/doc/html/draft-ietf-tls-esni