Feature: lsp-doctor to include check for lsp-java-java-path

[skybert]

Describe the bug
I spent hours debugging why Emacs failed to give me the full LSP experience. It only said:

make-process--with-editor-process-filter: Doing vfork: No such file or directory

I could find no log/error/message buffer that gave me a hint as to what file or directory that wasn't found. The way I found out, was to bisecting my .emacs.

I thus believe it would be a great improvement if lsp-doctor could check if the Java runtime could be found (when lsp-java has been loaded)

To Reproduce

;; insert the quick start guide .emacs snippet from the README.md

;; then, set a non-existing Java path
(setq lsp-java-java-path "/usr/lib/jvm/java-does-NOT-exist-openjdk/bin/java")

Expected behavior
The doctor should find what's wrong (granted lsp-java is loaded):

M-x lsp-doctor
..
lsp-java could find valid Java runtime: NOT (/usr/lib/jvm/java-does-NOT-exist-openjdk/bin/java does not exist, configure lsp-java-java-path to point to a valid java runtime).