add function handling (basic)

internal/worker/run.go

@@ -0,0 +1,25 @@
+package worker
+
+import (
+	"bytes"
+	"fmt"
+	"os/exec"
+)
+
+func RunFunction(path string, fc *FuncConfig, input []byte) ([]byte, error) {
+	if fc.Runtime != "exec" {
+		return nil, fmt.Errorf("unsupported runtime: %s", fc.Runtime)
+	}
+
+	cmd := exec.Command(path)
+	cmd.Stdin = bytes.NewReader(input)
+
+	var out bytes.Buffer
+	cmd.Stdout = &out
+	cmd.Stderr = &out
+
+	if err := cmd.Run(); err != nil {
+		return nil, fmt.Errorf("failed: %w\noutput: %s", err, out.String())
+	}
+	return out.Bytes(), nil
+}