Implement HTTPS JSON UA transport

[marcschier]

Implement HTTPS JSON UA transport

The OPC UA .NET Standard Stack currently does not support the HTTPS
JSON transport profile
(http://opcfoundation.org/UA-Profile/Transport/https-uajson), even
though the URI constant exists in
SecurityConstants.HttpsJsonTransport.
Today only https-uabinary (binary-over-HTTPS) is wired up in
Opc.Ua.Bindings.Https.

Acceptance criteria

• Both client and server can negotiate and run an OPC UA session over
  HTTPS with OPC UA JSON message encoding, end-to-end, using the same
  endpoint discovery / security policy / user identity flows as the
  existing https-uabinary transport.
• Integration tests for request / response, fault paths, certificate
  validation, and JSON encoder / decoder corner cases.
• Docs/Profiles.md is updated to list https-uajson as supported and
  the "Currently not supported" entry is removed.
• The CTT certification report covers the HTTPS-JSON profile.

Notes

• Likely lives alongside the existing binary path in
  Opc.Ua.Bindings.Https.
• The JSON encoder / decoder pipeline already exists in
  Stack/Opc.Ua.Types/Encoders/ and is exercised by the PubSub MQTT
  JSON path; the work here is the HTTPS transport adapter.
• Tracked separately from WebSocket Secure (see companion issue).