Async functionality

[clehner]

Bindings should be async where possible.

Current status of what in DIDKit supports async usage

... with notes and pointers to relevant issues/PRs:

• [x] HTTP
• [x] python
• [x] web (WASM)
• [ ] Dart/Flutter - uses C. #283
• [ ] Go - uses C
• [ ] CLI - n/a?
• [ ] C - can use threads?
• [ ] Java (JNI)
• [ ] node (Node.js)
• [ ] iOS (Swift) - uses C