Tls13KeySchedule.hkdfExpandLabel

ubyte[] hkdfExpandLabel(const(ubyte)[]  secret,  string label,
          const(ubyte)[]  context,  size_t length)

HKDF-Expand-Label as defined in RFC 8446 §7.1.

HkdfLabel = HkdfLabel { length: uint16, label: opaque vector ("tls13 " + label), context: opaque vector }

`secret`	The input secret.
`label`	The label string (without "tls13 " prefix).
`context`	The context data (transcript hash), or null.
`length`	Output length in bytes.

Derived key material.