Provide Mechanism to Access Protocol Version / API Version

[Carter12s]

I'm maintaining roslibrust with a major goal being maintaining ROS1 and ROS2 compatibility.

Happy for the API to continue to evolve, but it would be really handy to have some mechanism to discover what rosbridge_server variant I'm talking with, and adapt my protocol appropriately.

Providing this a ros service, "/rosbridge_server/get_version" that returns the version from the package.xml would be a simple way to do this. But we may prefer "/rosbridge_server/protocol_version" that returns a SemVer number corresponding to the design of the ROSBRIDGE_PROTOCOL.md

Versioning the protocol would be useful for the documentation as well!

[EzraBrooks]

Personally I'd propose hosting this as an HTTP GET endpoint on the same HTTP socket as the bridge hosts its WebSocket endpoint on.

I'd also like to see similar for "is the bridge in BSON mode" if we continue supporting BSON. Right now clients are blind as to whether they should be speaking BSON for every transaction or not - and if they try to speak JSON to a BSON-only bridge everything just explodes

[Carter12s]

Yeah would make sense to have this operate as standalone super simple protocol, so that you don't have to have the plumbing for making a service_request working ahead of this call. A simple http GET, that returns a little json payload would be perfect.

{
  server_version: "0.2.0", // Could do 3 numbers instead of string?
  api_version: "1.2.0", 
  supports_json: true,
  supports_cbor: true,
  supports_bson: false,
  supports_tls: false
}