x/tools/gopls: rethink serverDispatcher.ExecuteCommand

[adonovan]

In CL https://go-review.git.corp.google.com/c/tools/+/505875 I would have added this comment in the implementation of serverDispatcher.ExecuteCommand, but I can't because it's generated code.

// TODO(adonovan): allow the caller to pass in the result
// pointer so they can avoid an unnecessary JSON-to-any
// decoding and re-encoding to JSON before finally decoding to
// the correct Go struct type. (Also: the specific result
// type depends on the implementation of ExecuteCommand:
// (*Server).ExecuteCommand returns the specific Go type!
// This is not a meaningful interface.)

This issue is a reminder to clean it up.