Add a file-system backend for the certificate chain cache

[jpraynaud]

Why

The in-memory cache does not survive a restart, and the CLI starts a fresh process on every command, so nothing is saved across invocations.

What

Add a FileCertificateVerifierCache implementation of CertificateVerifierCache in mithril-client for non-WASM targets.

How

• Store each certificate in a separate file
• Use a folder for the committed cache
• Use a folder by certificate chain validation id for staging (support pruning?)
• Invalidate the cache entry upon parsing error of a certificate file
• Upon committing, the staged certificates are moved the to the committed cache (atomic files move, do we need some Mutex to commit staging area sequentially?)