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_unix.go

@@ -0,0 +1,45 @@
+// +build !windows
+
+package rpcplugin
+
+import (
+	"context"
+	"io"
+	"os"
+	"os/exec"
+)
+
+type process struct {
+	command *exec.Cmd
+}
+
+func newProcess(ctx context.Context, path string) (Process, io.ReadWriteCloser, error) {
+	ipc, childFiles, err := NewIPC()
+	if err != nil {
+		return nil, nil, err
+	}
+	defer childFiles[0].Close()
+	defer childFiles[1].Close()
+
+	cmd := exec.CommandContext(ctx, path)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	cmd.ExtraFiles = childFiles
+	err = cmd.Start()
+	if err != nil {
+		ipc.Close()
+		return nil, nil, err
+	}
+
+	return &process{
+		command: cmd,
+	}, ipc, nil
+}
+
+func (p *process) Wait() error {
+	return p.command.Wait()
+}
+
+func inheritedProcessIPC() (io.ReadWriteCloser, error) {
+	return InheritedIPC(3, 4)
+}