Add KV helper methods to Helpers interface (#11307)

* Add KV helper methods to Helpers interface

* Address code-review comments, add unit tests

* Update documentation for KVCompareAndSetJSON.

* Update assertions for helpers_bots_test.go

* Fix assertion not called name in test.

plugin/helpers.go

@@ -6,10 +6,26 @@ package plugin
 import "github.com/mattermost/mattermost-server/model"
 
 type Helpers interface {
-	// EnsureBot ether returns an existing bot user or creates a bot user with
+	// EnsureBot either returns an existing bot user or creates a bot user with
 	// the specifications of the passed bot.
 	// Returns the id of the bot created or existing.
 	EnsureBot(bot *model.Bot) (string, error)
+
+	// KVGetJSON retrievs a value based on the key.
+	KVGetJSON(key string, value interface{}) error
+
+	// KVSetJSON stores a key-value pair.
+	KVSetJSON(key string, value interface{}) error
+
+	// KVCompareAndSetJSON updates a key-value pair if the current
+	// value is equal to oldValue.
+	// Returns (false, err) if DB/marshal/unmarshal error occurred
+	// Returns (false, nil) if current value != old value
+	// Returns (true, nil) if current value == old value or new key is inserted
+	KVCompareAndSetJSON(key string, oldValue interface{}, newValue interface{}) (bool, error)
+
+	// KVSetWithExpiryJSON stores a key-value pair with an expiry time.
+	KVSetWithExpiryJSON(key string, value interface{}, expireInSeconds int64) error
 }
 
 type HelpersImpl struct {