Adds lage-server binary - buildxl breakaway process part 1

[kenotron]

In this change, we add a lage-server binary to lage. It is a new command that work like this:

1. lage-server launches cli/server.js
2. the tool will find any existing servers - if it doesn't find it, it'll create one given listening host & port
3. the tool will then execute what was given to lage-server (much like lage run)
4. it will stay in the background for some period of time as specified by --timeout (in seconds)
5. any lage-server run will ping for the server... if it finds a server, it'll actually act as a client passing along the execution args to the server.
6. a running server in the background would handle these without worrying about target graph dependencies

Part 2 will include these:

1. a way for the tasks to report which files were touched (read / write, etc.)
2. a way for the client to then probe / touch the files, so BuildXL integration can register those as simulated reads and writes... this helps with BXL's sandbox -> hash calculations for the caching mechanism

Part 3 will include these:

1. modify the info command to use "lage exec" or "lage-server" where appropriate