feat: Allow managedplugins to control their environment (#246)

This change allows callers of managedplugin to optionally pass in an `Environment` array, which, if used, will replace the default os.Environ for the created plugin.

The idea is that we will use this from the CLI to isolate plugin environment from one another. The core logic of how exactly that happens will stay in the CLI.

managedplugin/plugin.go

@@ -70,10 +70,11 @@ func (p PluginType) String() string {
 type Clients []*Client
 
 type Config struct {
-	Name     string
-	Registry Registry
-	Path     string
-	Version  string
+	Name        string
+	Registry    Registry
+	Path        string
+	Version     string
+	Environment []string // environment variables to pass to the plugin in key=value format
 }
 
 type Client struct {
@@ -276,6 +277,7 @@ func (c *Client) startDockerPlugin(ctx context.Context, configPath string) error
 		Image: configPath,
 		Cmd:   pluginArgs,
 		Tty:   true,
+		Env:   c.config.Environment,
 	}
 	hostConfig := &container.HostConfig{
 		PortBindings: map[nat.Port][]nat.PortBinding{
@@ -393,6 +395,9 @@ func (c *Client) startLocal(ctx context.Context, path string) error {
 		return fmt.Errorf("failed to get stdout pipe: %w", err)
 	}
 	cmd.Stderr = os.Stderr
+	if c.config.Environment != nil {
+		cmd.Env = c.config.Environment
+	}
 	cmd.SysProcAttr = getSysProcAttr()
 	if err := cmd.Start(); err != nil {
 		return fmt.Errorf("failed to start plugin %s: %w", path, err)