feat: Image / media uploads `@ts-ghost/admin-api`

[PhilDL]

Describe the feature you'd like to request

To be feature complete @ts-ghost/admin-api should handle upload of:

• images
• media
• files
• theme + theme activation

Describe the solution you'd like to see

There should be new resource on the API object with a single method upload.

Create a new type of fetcher.

Describe alternate solutions

Get inspiration from the original ghost sdk

Additional information

No response

👨‍👧‍👦 Contributing

• [X] 🙋‍♂️ Yes, I'd be down to file a PR implementing this feature!