feat: use OS conform path for storing cached results

Instead of storing cached values in the config yaml, they are now stored
under these OS specific locations:
* Linux: `~/.cache/k8sgpt`
* MacOS: `~/Library/Caches`
* Windows: `%LocalAppData%\cache`

Additionally a `Cache` package and interface has been introduced.
Currently there are two implementations:
* Noop - Doesn't do anything
* FileBased - Stores data in files under the locations listed above

fixes #323

Signed-off-by: Patrick Pichler <git@patrickpichler.dev>

pkg/cache/file_based.go

@@ -0,0 +1,58 @@
+package cache
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/adrg/xdg"
+	"github.com/k8sgpt-ai/k8sgpt/pkg/util"
+)
+
+var _ (ICache) = (*FileBasedCache)(nil)
+
+type FileBasedCache struct{}
+
+func (*FileBasedCache) Exists(key string) bool {
+	path, err := xdg.CacheFile(filepath.Join("k8sgpt", key))
+
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "warning: error while testing if cache key exists:", err)
+		return false
+	}
+
+	exists, err := util.FileExists(path)
+
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "warning: error while testing if cache key exists:", err)
+		return false
+	}
+
+	return exists
+}
+
+func (*FileBasedCache) Load(key string) (string, error) {
+	path, err := xdg.CacheFile(filepath.Join("k8sgpt", key))
+
+	if err != nil {
+		return "", err
+	}
+
+	data, err := os.ReadFile(path)
+
+	if err != nil {
+		return "", err
+	}
+
+	return string(data), nil
+}
+
+func (*FileBasedCache) Store(key string, data string) error {
+	path, err := xdg.CacheFile(filepath.Join("k8sgpt", key))
+
+	if err != nil {
+		return err
+	}
+
+	return os.WriteFile(path, []byte(data), 0600)
+}