Add the ability to know the OSM server via Remote Control

[TrickyFoxy]

I added an osm_server_url field to the /version command response with the user's current OSM server.

{
  "protocolversion": {
    "major": 1,
    "minor": 13
  },
  "application": "JOSM RemoteControl",
  "version": 19402,
  "osm_server_url": "https://api.openstreetmap.org/api"
}

I am writing an extension that makes heavy use of Remote Control in JOSM. For example, to revert changesets or open objects in JOSM. The extension supports working with the API sandbox, but the user may forget to change the OSM server in the settings. [1] In the best case, an error will occur, in the worst case, you will corrupt the user's layer or, even worse, he will not notice this and send the changes to the server.

Therefore, I propose to give external tools the ability to check the current OSM server. Since most users do not change this setting, I do not see any risk of disclosing private information.

Patch note: I replaced the static field with a static method because the OSM server can be modified while JOSM is running.

[1] ​https://github.com/deevroman/better-osm-org/issues/258