refactor providers

[sneakers-the-rat]

providers are currently all over the map, one of the last major things that needs to be fixed. Their design is, dare i say, batshit and bullshit. and i am allowed to say that because i designed them.

What we want:

• A way of abstracting getting models..
• from a variety of sources
• on-demand (ie. we might only have the NWB schema and need to generate the linkml and pydantic models)
• with correctly resolved versions

TODO:

• just build into nwb_models or else use it for the namespace everywhere
• split out sources into separate classes, switching between the builtin models and cached models is messy as hell
• use the ectopic model finder to replace much of the pydantic provider, ideally the interface would just be to from nwb_models.{namespace}.{version}.{schema} import {class}
• make add_schema methods rather than build, which make for clearer semantics - when loading an nwb file, add all the models we don't have to the cache directory, and then we can just access them forever.