Add support for maintenance info in instance provisioning requests bodies

## Description

Support feature from OSB spec: [maintenance info](https://github.com/openservicebrokerapi/servicebroker/blob/master/spec.md#maintenance-info-object)

## Implementation

Add an attribute of type `MaintenanceInfo` in structs `ProvisionInstanceRequest`, `UpdateInstanceRequest`, `GetInstanceResponse`, `PreviousValues`.