Secrets: Implement secrets manager plugin fallback store (#54496)

* Refactor fallback to be isolated to plugin secret store

* Check for error value on replace fallback test helper

* Move ResetPlugin from test_helpers.go to plugin.go

* Add check to GetUnwrappedStoreFromCache

* Add fallback GetAll query to WithFallbackEnabled

* Add mutex lock to WithFallbackEnabled

* Add cache to fallback store

* Fix linter issues

* Fix linter issues

* Fix linter issues

pkg/services/secrets/kvstore/sql.go

@@ -3,7 +3,6 @@ package kvstore
 import (
 	"context"
 	"encoding/base64"
-	"errors"
 	"sync"
 	"time"
 
@@ -30,10 +29,7 @@ type cachedDecrypted struct {
 	value   string
 }
 
-var (
-	b64                   = base64.RawStdEncoding
-	errFallbackNotAllowed = errors.New("fallback not allowed for sql secret store")
-)
+var b64 = base64.RawStdEncoding
 
 func NewSQLSecretsKVStore(sqlStore sqlstore.Store, secretsService secrets.Service, logger log.Logger) *SecretsKVStoreSQL {
 	return &SecretsKVStoreSQL{
@@ -244,14 +240,6 @@ func (kv *SecretsKVStoreSQL) GetAll(ctx context.Context) ([]Item, error) {
 	return items, err
 }
 
-func (kv *SecretsKVStoreSQL) Fallback() SecretsKVStore {
-	return nil
-}
-
-func (kv *SecretsKVStoreSQL) SetFallback(_ SecretsKVStore) error {
-	return errFallbackNotAllowed
-}
-
 func (kv *SecretsKVStoreSQL) getDecryptedValue(ctx context.Context, item Item) ([]byte, error) {
 	kv.decryptionCache.Lock()
 	defer kv.decryptionCache.Unlock()