v4.1.0

Add new struct fields and errors to enable app developers to perform maintenance on service instances, as described in: openservicebrokerapi/servicebroker#628

Specifically:

• ProvisionDetails and UpdateDetails have an optional MaintenanceInfo field
• ErrMaintenanceInfoConflict and ErrMaintenanceInfoNilConflict to be returned when the provided maintenance info does not match the current catalog

Also adds instances_retrievable/bindings_retrievable catalog fields added to the spec in 2.14. Thanks to @drnic.