PLT-7407: Back-end plugin mechanism (#7177)

* begin backend plugin wip

* flesh out rpcplugin. everything done except for minor supervisor stubs

* done with basic plugin infrastructure

* simplify tests

* remove unused test lines

plugin/rpcplugin/process_windows.go

@@ -0,0 +1,17 @@
+package rpcplugin
+
+import (
+	"context"
+	"fmt"
+	"io"
+)
+
+func newProcess(ctx context.Context, path string) (Process, io.ReadWriteCloser, error) {
+	// TODO
+	return nil, nil, fmt.Errorf("not yet supported")
+}
+
+func inheritedProcessIPC() (*IPC, error) {
+	// TODO
+	return nil, fmt.Errorf("not yet supported")
+}