Implement SPICE-0009 External Readers (#660)

This adds a new feature, which allows Pkl to read resources and modules from external processes.

Follows the design laid out in SPICE-0009.

Also, this moves most of the messaging API into pkl-core

pkl-core/src/test/resources/org/pkl/core/project/project1/PklProject

@@ -22,4 +22,22 @@ evaluatorSettings {
   noCache = false
   rootDir = "my-root-dir/"
   timeout = 5.min
+  externalModuleReaders {
+    ["scheme1"] {
+      executable = "reader1"
+    }
+    ["scheme2"] {
+      executable = "reader2"
+      arguments { "with"; "args" }
+    }
+  }
+  externalResourceReaders {
+    ["scheme3"] {
+      executable = "reader3"
+    }
+    ["scheme4"] {
+      executable = "reader4"
+      arguments { "with"; "args" }
+    }
+  }
 }