Add package model

[helrond]

Is your feature request related to a problem? Please describe.

All of the microservice applications have a model which defines a basic package (i.e. a bag plus some metadata). These are all named slightly different things (SIP, Transfer, Bag, etc) but conceptually they are all very similar and share a number of fields.

Describe the solution you'd like

Create a Package model in asterism. This will likely consist of several steps:

• Survey microservice applications to identify common model fields
• Create a new model in asterism with the appropriate required and optional fields
• Cut a new release of asterism
• replace existing models in microservice applications with imports