V2 - Extract API structs into separate library

[jcace]

Description: Allow 3rd parties using Go to easily import the interface types for nice unmarshal/marshalling

see: https://github.com/data-preservation-programs/go-spade-apitypes

Could keep it in the same repo, but needs to be a separate package

ETA: YYY-MM-DD
Maintainer:

### Milestones
- [ ] <add here>

Known dependencies:

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