Introduce a printer interface for flux resource

If implemented, there will a common interface to print flux resource.

We are adding new way to print resource information e.g. diff of
objects.

Signed-off-by: Soule BA <soule@weave.works>

pkg/printers/dyff.go

@@ -0,0 +1,56 @@
+/*
+Copyright 2022 The Flux authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package printers
+
+import (
+	"fmt"
+	"io"
+
+	"github.com/homeport/dyff/pkg/dyff"
+)
+
+// DyffPrinter is a printer that prints dyff reports.
+type DyffPrinter struct {
+	OmitHeader bool
+}
+
+// NewDyffPrinter returns a new DyffPrinter.
+func NewDyffPrinter() *DyffPrinter {
+	return &DyffPrinter{
+		OmitHeader: true,
+	}
+}
+
+// Print prints the given args to the given writer.
+func (p *DyffPrinter) Print(w io.Writer, args ...interface{}) error {
+	for _, arg := range args {
+		switch arg := arg.(type) {
+		case dyff.Report:
+			reportWriter := &dyff.HumanReport{
+				Report:     arg,
+				OmitHeader: p.OmitHeader,
+			}
+
+			if err := reportWriter.WriteReport(w); err != nil {
+				return fmt.Errorf("failed to print report: %w", err)
+			}
+		default:
+			return fmt.Errorf("unsupported type %T", arg)
+		}
+	}
+	return nil
+}