Implement ClientHelloSpec JSON Unmarshaler (#176)

* wip: staging work

* wip: staging work

* feat: ClientHello JSON Unmarshaler

Allowing unmarshalling a JSON object into a ClientHelloSpec.

* feat: ClientHello JSON Unmarshaler rev

- Revised JSON ClientHello format
- Implemented `TLSExtensionJSON` interface for some more extensions

u_public.go

@@ -543,8 +543,8 @@ func (fh *finishedHash) getPublicObj() FinishedHash {
 
 // TLS 1.3 Key Share. See RFC 8446, Section 4.2.8.
 type KeyShare struct {
-	Group CurveID
-	Data  []byte
+	Group CurveID `json:"group"`
+	Data  []byte  `json:"key_exchange,omitempty"` // optional
 }
 
 type KeyShares []KeyShare
@@ -568,8 +568,8 @@ func (KSS KeyShares) ToPrivate() []keyShare {
 // TLS 1.3 PSK Identity. Can be a Session Ticket, or a reference to a saved
 // session. See RFC 8446, Section 4.2.11.
 type PskIdentity struct {
-	Label               []byte
-	ObfuscatedTicketAge uint32
+	Label               []byte `json:"identity"`
+	ObfuscatedTicketAge uint32 `json:"obfuscated_ticket_age"`
 }
 
 type PskIdentities []PskIdentity