Support attach/update sources

[yyoncho]

See eclipse-jdtls/eclipse.jdt.ls#837

[mahinshaw]

Could we do this with a var? And then a user could add a dir-local entry to assign the value per project/workspace?

At some point a mechanism for doing this interactively could be added.

[yyoncho]

At this point dir locals does not work : emacs-lsp/lsp-mode#405 . I am not familiar what is the API so I am not sure whether it will work and what should be the end state. I am pro any solution that will move us closer to the target state.

[mahinshaw]

Ok. Good to know. I'll see if there is anything else to hook into that would solve this on a per project basis.