Handle cache for certificate verification in WASM client

[jpraynaud]

Why

We want the client library to provide faster certificate verification in the explorer.

What

Provide a list of already signed transactions to the client so that it does not verify them or handle a dedicated browser store for that purpose. It's worth mentioning that once a certificate is verified and cache, all the remaining certificate chain is considered valid.We don't know yet if implementing a cache in the local storage of the browser is a feature that should be activated for all the users of the Mithril client (explorer, lace, ...). We should probably keep this under a unstable or specific feature tag.

How

• Handle cache for certificates verification by the client
• Adapt explorer to take advantage of these new capabilities?