feat: Allow building a version of the plugin that embeds the javaparser server

[blorente]

Thus allowing it to be "statically linkable".

The user-facing UX has two options:

• To use the command line, like this: bazel build //:<your-gazelle> '--@contrib_rules_jvm//java/gazelle:embed_server=True'
• To create a transition, like the one we use for testing in java/gazelle/testdata/gazelle_build_mode.bzl.

Caveats and implementation notes:

• We always need a java binary at runtime. This can be specified via JAVA_HOME, or a new environment variable, GAZELLE_JAVA_BIN. We don't strictly need the new env var, we should be able to always use JAVA_HOME, but I thought it would be nice to have a variable explicitly for the Java plugin, just in case it differs from your runtime environment.
• We implement the embedding directly in servermanager.go, by conditionally compiling one of two versions of the library.
• We need to do some hacks for testing, because we need to wrap the actual gazelle binary, and the tests do not run the gazelle binary from Bazel's sandboxed working directory.
• When running the embedded server, we materialize the javaparser server jar 100% of the time. We could be smarter about this, but the server is small enough that I don't think it matters.

[stevebarrau]

The approach look reasonable.

Code level comments only.

[shs96c]

LGTM