This implements JSON-based port file. Thoughout the lifetime of the sbt server there will be `cwd / "project" / "target" / "active.json"`, which contains `url` field.
Using this `url` the potential client, such as IDEs can find out which port number to hit.
Ref #3508
LogManager implementation is modified to use ManagedLogger, which can swap out backing Appenders without re-creating the log instance.
The State was also changed to track `currentCommand: Option[Exec]`. `Exec` knows the origin of the command invocation, and using that we can now send the network-originated events only to the network clients.
Combined together, this implements log splitting between the sbt clients (channels).
This is the beginning of a lightweight client, which talks to the
server over Contraband-generated JSON API. Given that the server is
started on port 5173:
```
$ cd /tmp/bogus
$ sbt client localhost:5173
> compile
StatusEvent(Processing, Vector(compile, server))
StatusEvent(Ready, Vector())
StatusEvent(Processing, Vector(, server))
StatusEvent(Ready, Vector())
```