More customizable eclipse jdt configuration

[senatoreg]

Some changes to make Eclipse Jdt configuration more customizable. Particularly two variables to specify configuration directory and java home to be used to start Eclipse Jdt LS. I think they could be useful for example, in case of system installation of Eclipse JDT LS, which is my case of course.

Let me know.

[joaotavora]

Some changes to make Eclipse Jdt configuration more customizable

Is it so that one can launch somehow launch Eclipse Jdt in the project's home directory with some arguments and it will start working? Or is it still the case that one has to custom tailor the java command very tightly?

If the former, than we can start to to dismantle the Eclipse-specific function and move it into a nicer-behaving implementation base solely on eglot-server-programs.

If the latter case, these defcustoms are not going into eglot.el, so either:

1. you put in a little work and implement Add convenience wrappers for starting the server from the command line eclipse-jdtls/eclipse.jdt.ls#748, I can't right now, but it shouldn't be very hard
2. you create an eglot-eclipse.el library until someone creates 1.