C++: add missing implementations for client advertised schema/encoding (

#876)

### Changelog

C++ server/client: add missing implementations for client advertised
schema / schema encoding

### Docs

None

### Description

Although foxglove ws protocol allow client to advertise complete schema
definition string as well as schema name, current C++ implementation
lacks such fields. Particularly:
* `ClientAdvertisement` struct lacks `schemaEncoding` field,
* `ClientAdvertisement` struct has `schema` field, but it is never
populated (websocket server does not read schema or schema encoding to
json)
* websocket client is does not serialize schema or schema encoding to
json

This PR resolves above problems, and additionally since this is breaking
change from unimplemented to implemented, types are decided as
following:
* `schema` field type is changed from `std::vector<uint8_t>` to
`std::optional<std::string>`, as schema fields are passed in as json
quoted string, and it's optional
* `schemaEncoding` field type is determined as
`std::optional<std::string>`, the same reason for `schema`.


Fixes: #875
Fixes: FG-9447

---------

Co-authored-by: Hans-Joachim Krauch <achim-k@users.noreply.github.com>

cpp/foxglove-websocket/include/foxglove/websocket/common.hpp

@@ -79,7 +79,8 @@ struct ClientAdvertisement {
   std::string topic;
   std::string encoding;
   std::string schemaName;
-  std::vector<uint8_t> schema;
+  std::optional<std::string> schema;
+  std::optional<std::string> schemaEncoding;
 };
 
 struct ClientMessage {

cpp/foxglove-websocket/include/foxglove/websocket/websocket_client.hpp

@@ -23,6 +23,12 @@ inline void to_json(nlohmann::json& j, const ClientAdvertisement& p) {
                      {"topic", p.topic},
                      {"encoding", p.encoding},
                      {"schemaName", p.schemaName}};
+  if (p.schema) {
+    j["schema"] = p.schema.value();
+  }
+  if (p.schemaEncoding) {
+    j["schemaEncoding"] = p.schemaEncoding.value();
+  }
 }
 
 using TextMessageHandler = std::function<void(const std::string&)>;

cpp/foxglove-websocket/include/foxglove/websocket/websocket_server.hpp

@@ -1340,6 +1340,12 @@ void Server<ServerConfiguration>::handleAdvertise(const nlohmann::json& payload,
     advertisement.encoding = chan.at("encoding").get<std::string>();
     advertisement.schemaName = chan.at("schemaName").get<std::string>();
 
+    if (const auto it = chan.find("schema"); it != chan.end()) {
+      advertisement.schema = it->get<std::string>();
+    }
+    if (const auto it = chan.find("schemaEncoding"); it != chan.end()) {
+      advertisement.schemaEncoding = it->get<std::string>();
+    }
     clientPublications.emplace(channelId, advertisement);
     {
       std::unique_lock<std::shared_mutex> clientsLock(_clientsMutex);