Handle cancellation of certificate verification in WASM client

## Why
We want the client library to allow for cancellation of an in progress verification

## What
Implement a mechanism to detect a cancellation request from the browser and attempt to stop the certificate verification in that case.

## How

- [ ] Handle cancellation request for certificates verification (see [AbortController signal property](https://developer.mozilla.org/en-US/docs/Web/API/AbortController/signal))
- [ ] Adapt explorer to take advantage of these new capabilities
- [ ] Refactor duplicate `CertificateChainVerifier`?
- [ ] Update documentation