[design] Make GA API plan

[BigLep]

Done Criteria

We're doing an “architectural lens” pass with on what the public Filecoin Pin CLI and SDK affordance looks like to external developers to decide what the “public, stable surface” is for GA. #279 (or a linked issue) has been updated with the scope of the changes we're going to make to for GA.

Why Important

See #279

Notes

• We have to be pragmatic and go after the "must-dos" or "lower effort / higher impact" items. This isn't a "boil the ocean" endeavor.
• Related issues where API improvements have been discussed:
  • feat: migrate callback API to onProgress with typed responses #91
  • backport: remove all custom class instantiation of synapse-sdk classes #278