Handle cancellation of certificate verification in WASM client

[jpraynaud]

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)
• [ ] Adapt explorer to take advantage of these new capabilities
• [ ] Refactor duplicate CertificateChainVerifier?
• [ ] Update documentation