feature(thumbnails): add the ability to define custom image processors (#7409)

* feature(thumbnails): add the ability to define custom image processors

* fix(ci): add exported member comment

* docs(thumbnails): mention processors in readme

* fix: codacy and code review feedback

* fix: thumbnail readme markdown

Co-authored-by: Martin <github@diemattels.at>

---------

Co-authored-by: Martin <github@diemattels.at>

services/thumbnails/pkg/thumbnail/storage/filesystem_test.go

@@ -0,0 +1,65 @@
+package storage_test
+
+import (
+	"image"
+	"testing"
+
+	tAssert "github.com/stretchr/testify/assert"
+
+	"github.com/owncloud/ocis/v2/services/thumbnails/pkg/thumbnail/storage"
+)
+
+func TestFileSystem_BuildKey(t *testing.T) {
+	tests := []struct {
+		r    storage.Request
+		want string
+	}{
+		{
+			r: storage.Request{
+				Checksum: "120EA8A25E5D487BF68B5F7096440019",
+				Types:    []string{"png", "jpg"},
+				Resolution: image.Rectangle{
+					Min: image.Point{
+						X: 1,
+						Y: 2,
+					},
+					Max: image.Point{
+						X: 3,
+						Y: 4,
+					},
+				},
+				Characteristic: "",
+			},
+			want: "12/0E/A8A25E5D487BF68B5F7096440019/2x2.png",
+		},
+		{
+			r: storage.Request{
+				Checksum: "120EA8A25E5D487BF68B5F7096440019",
+				Types:    []string{"png", "jpg"},
+				Resolution: image.Rectangle{
+					Min: image.Point{
+						X: 1,
+						Y: 2,
+					},
+					Max: image.Point{
+						X: 3,
+						Y: 4,
+					},
+				},
+				Characteristic: "fill",
+			},
+			want: "12/0E/A8A25E5D487BF68B5F7096440019/2x2-fill.png",
+		},
+	}
+
+	s := storage.FileSystem{}
+	assert := tAssert.New(t)
+
+	for _, tt := range tests {
+		tt := tt
+		t.Run("", func(t *testing.T) {
+			assert.Equal(s.BuildKey(tt.r), tt.want)
+		})
+	}
+
+}