first prototype of the thumbnail service

Currently uses in memory caching and loads the file from the local
filesystem.

pkg/thumbnails/cache/cache.go

@@ -0,0 +1,11 @@
+package cache
+
+import (
+	"image"
+)
+
+// Cache defines the interface for a thumbnail cache.
+type Cache interface {
+	Get(key string) image.Image
+	Set(key string, thumbnail image.Image) (image.Image, error)
+}

pkg/thumbnails/cache/filesystem.go

@@ -0,0 +1,3 @@
+package cache
+
+// TODO: implement filesystem cache for longer persistence

pkg/thumbnails/cache/inmemory.go

@@ -0,0 +1,22 @@
+package cache
+
+import "image"
+
+func NewInMemoryCache() InMemoryCache {
+	return InMemoryCache{
+		store: make(map[string]image.Image),
+	}
+}
+
+type InMemoryCache struct {
+	store map[string]image.Image
+}
+
+func (fsc InMemoryCache) Get(key string) image.Image {
+	return fsc.store[key]
+}
+
+func (fsc InMemoryCache) Set(key string, thumbnail image.Image) (image.Image, error) {
+	fsc.store[key] = thumbnail
+	return thumbnail, nil
+}