Add meta information to protobuf observation

[rk1a]

Information that might be useful to have:

• absolute mouse positions
• mouse wheel state
• pitch and yaw of camera
• whether a menu is open
• time stamp
• any new chat messages
• inventory content

A draft of such a protobuf object is here: https://github.com/EleutherAI/minetest/blob/develop/proto/objects.proto#L107

A more flexible approach would be to outsource this to mods.
For example, define a global table variable INFO that is being read out by the Minetest client (like i the case for REWARD and TERMINAL).
Then we could add any information that is available to mods into this table (although for some things, like inventory, this requires setting up mod channels to the server)