Add HTTP signature to GET requests to support Mastodon's AUTHORIZED_FETCH setting.

[pburke]

• Add signedFetch() utility function.
• Add signedGetJSON() and signedPostJSON() helper functions.
• Refactor signAndSend() function to use it.
• Replace calls to fetch() with signedGetJSON() or signedPostJSON() as needed.

Fixes #58

[pburke]

I tested these changes on a local Postmarks install interacting with a local Mastodon install. More testing against live servers is probably needed.

[ckolderup]

I'll hold off on a full review since I agree with your decision on the exported functions, but this is a great start, thank you.

[pburke]

Added signedGetJSON and signedPostJSON helpers, so I think this is ready for a full review! :sparkles: