Clean up Client API

Nobody has claimed this yet.

It has grown organically so we could test JS/Rust SDK. It's now time to think about the API more before we open it up to everyone.

Thoughts on each part of the API:
- `ClientCreationOpts` : Addition of extra options in https://github.com/matrix-org/complement-crypto/pull/144 keeps it extensible. We should cut down on as many fields as possible.
- [X] `Must...` variants: Remove them and decorate the Client API in tests with `Must` variants. It's crazy that impls need to implement `MustStartSyncing` AND `StartSyncing`, when the caller can just do the `err != nil` check on their end. https://github.com/matrix-org/complement-crypto/pull/146
- `Event`: re-check rust SDK and try to make it match the wire format for events as much as possible.
- `GetNotification`: it's useful as other SDKs have push-notification-like code path, but it's very rust SDK specific currently.

RPC-wise:
- `RequestOwnUserVerification` and `ListenForVerificationRequests`: we need to implement them. Can probably poll like we do with the waiters?
- Check RPC-only functions for sanity:
* Server.WaiterStart
* Server.WaiterPoll

Probably worth having a version handshake initially along with the port (e.g `56434 v1`). If we don't, we may have missing RPC functions or missing fields etc. This means we need to document the RPC API. Can we automate it?

Start by inventorying the Client API areas named in the issue: ClientCreationOpts, Must... variants, Event, GetNotification, and the verification RPCs. Review the linked pull requests and the Server.WaiterStart/Server.WaiterPoll functions before deciding scope. Done requires an agreed API cleanup, implemented missing RPCs, and documented or versioned RPC compatibility.

Written by the indexing model from the issue text.