Add attested-tls crate

[ameba23]

This adds a the attested TLS crate which provides an attested certificate resolver, providing and handling renewal of TLS certificates with embedded attestations, and an attested certificate verifier, which can verify these attestations during the TLS handshake.

Note: This PR targets peg/add-attestation-crate as i needed stuff from that branch.

Attestation input data is computed as

SHA512(
  Der-encoded public key 
  || not before date as seconds big endian u64
  || not after date as seconds big endian u64
  || hostname
)

Features

• Both self-signed certificates and private certificate authorities are supported
• Verifier caches trusted certificates and skips subsequent attestation verification for them
• Attested client authentication supported

Things to be aware of

• Panics on poisoned mutex - which happens if a thread panics while holding the gaurd. If the code is correct this is acceptable because there should be no execution path in which this can happen.
• Attestation verifier currently blocks over an async call - see Calling async verify function from synchronous verify_server_cert function #2