Handle cache for certificate verification in WASM client

[jpraynaud]

Why

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

What

Implement certificate cache mechanism in the client library and use it in the client WASM.

How

• (De)activate the cache with a setting on the client WASM
• Use the certificate cache as unstable feature
• Define a CertificateCacher trait in mithril-client (with an expiration delay)
• Implement a LocalStorageCertificateCacher in the mithril-client-wasm
• Use the CertificateCacher in the verify_chain implementation of the MithrilCertificateVerifier in mithril-client
  • If a certificate is verified and cached, it is not downloaded and verified again (iif it is not the genesis certificate)
  • Verify the genesis certificate even if the cache is activated
• Adapt feedback receiver to give information about the certificate verification (cached or not)
• Verify security assumptions for browser local storage
• Adapt explorer with certificate caching
• Adapt examples with certificate caching