Provide maturity annotation for classes and fields

[jsstevenson]

GKS schemas often include maturity model annotations on objects and properties to indicate some expectation of model stability. It'd be nice to access this somehow within VRS-Python.

• Most naive solution: put it in the docstring. Not machine-readable, but simple to implement.
• Maybe the next simplest solution: include it as part of the model_json_schema() output. This is computationally accessible, albeit somewhat indirectly so. It would most directly match the schema itself.
• Implement as some kind of private or dunder field on each schema class, e.g. Allele.__maturity__ = <MaturityModel.TRIAL_USE>. Not sure of what the equivalent way to get property maturity would be, though.

Possible use cases/helper functions to implement

• A function that takes a Ga4ghIdentifiableObject and tells you which components or properties are most immature, i.e. are most liable to change in structure in future releases
• A function that takes a Ga4ghIdentifiableObject and tells you whether it contains any components or properties that are below a particular maturity model threshold and/or are deprecated