MM-8602: KV Store Json helper tweaks (#11502)

* move kv helpers to helpers_kv*.go

* change KVGetJSON return signature

Return a boolean and an error, to clearly indicate if no value was found and thus no value unmarshalled into the target interface.

Also fix an issue with CompareAndSet to allow an oldValue of nil (i.e.
expected to be unset).

* add missing license

* tweak documentation

* document KVSetWithExpiryJSON minimum version

plugin/helpers_bots.go

@@ -4,7 +4,6 @@
 package plugin
 
 import (
-	"encoding/json"
 	"time"
 
 	"github.com/mattermost/mattermost-server/model"
@@ -65,42 +64,3 @@ func (p *HelpersImpl) EnsureBot(bot *model.Bot) (retBotId string, retErr error)
 
 	return createdBot.UserId, nil
 }
-
-func (p *HelpersImpl) KVGetJSON(key string, value interface{}) error {
-	data, err := p.API.KVGet(key)
-	if err != nil {
-		return err
-	}
-
-	return json.Unmarshal(data, value)
-}
-
-func (p *HelpersImpl) KVSetJSON(key string, value interface{}) error {
-	data, err := json.Marshal(value)
-	if err != nil {
-		return err
-	}
-	return p.API.KVSet(key, data)
-}
-
-func (p *HelpersImpl) KVCompareAndSetJSON(key string, oldValue interface{}, newValue interface{}) (bool, error) {
-	oldData, err := json.Marshal(oldValue)
-	if err != nil {
-		return false, errors.Wrap(err, "unable to marshal old value")
-	}
-
-	newData, err := json.Marshal(newValue)
-	if err != nil {
-		return false, errors.Wrap(err, "unable to marshal new value")
-	}
-
-	return p.API.KVCompareAndSet(key, oldData, newData)
-}
-
-func (p *HelpersImpl) KVSetWithExpiryJSON(key string, value interface{}, expireInSeconds int64) error {
-	data, err := json.Marshal(value)
-	if err != nil {
-		return err
-	}
-	return p.API.KVSetWithExpiry(key, data, expireInSeconds)
-}