SDK types should match Stela types

[slifty]

The permanent SDK was attempting to move from the "API" types to a more standard type.

In parallel, the stela project started and likewise has attempted to create a cleaner type.

Unfortunately we now have two cleaner types (plus the third "legacy" type)! It would be great to lean into the stela types instead, updating this SDK API to match stela's API where possible.

[slifty]

As a related point: this issue speaks to need for a broader strategic question of the sdk's purpose and place.

We might want to move to a code-generated version of this SDK instead of the manually maintained one. Also, ideally the front-end code base would use the SDK when interacting with modern endpoints (v2 and stela).