[server] Enable multi-part upload flag for mobile (#2721)

## Description

## Tests

server/ente/remotestore.go

@@ -30,6 +30,7 @@ type FeatureFlagResponse struct {
 	RecoveryKeyVerified bool `json:"recoveryKeyVerified"`
 	InternalUser        bool `json:"internalUser"`
 	BetaUser            bool `json:"betaUser"`
+	EnableMobMultiPart  bool `json:"enableMobMultiPart"`
 }
 
 type FlagKey string

server/pkg/controller/remotestore/controller.go

@@ -55,6 +55,10 @@ func (c *Controller) GetFeatureFlags(ctx *gin.Context) (*ente.FeatureFlagRespons
 	response := &ente.FeatureFlagResponse{
 		EnableStripe:    true, // enable stripe for all
 		DisableCFWorker: false,
+		// When true, users will see an option to enable multiple part upload in the app
+		// Changing it to false will hide the option and disable multi part upload for everyone
+		// except internal user.rt
+		EnableMobMultiPart: true,
 	}
 	for key, value := range values {
 		flag := ente.FlagKey(key)