Add attestation crate

[ameba23]

This adds a crate with the attestation generation, verification, and measurement handling logic refactored out of attested-tls-proxy.

We may not need the dcap module - as this can be handled internally by ra-tls. But because the dcap stuff is also internally used by the azure attestation, we need to keep it in for now.

Paired PR

This pairs with flashbots/attested-tls-proxy#150 which uses this branch for attestation crate, to avoid duplicating the code.

Eventually we will probably not need that repo anymore, but as long as it is still being used by Buildernet we have to maintain it and it makes sense for the attestation code to be in just one place.

Github workflow for testing, clippy and formatting

I added a workflow for testing. I think this is an opinionated way of doing things, and maybe doesn't belong in this PR. But i wanted to see it pass before putting this up for review. So we can always switch to another CI script in a followup afterwards.

This also adds a rust-toolchain.toml using nightly which i needed locally to get the formatting correct for CI.

Crypto provider for rustls

When used as a library we have default-features = false for tokio-rustls, to allow the caller to choose crypto provider. As a dev-dependency we use the default provider for testing.