feat: wasi-crypto as an optional wasi feature

[rjzak]

Wasi-Crypto gated behind the wasi-crypto feature.

• Needs the PR for SHA-384 hashing support feat: add sha-384 hashing WebAssembly/wasi-crypto#80
• RSA public key parsing in spki.rs likely needs help
• spki.rs fails to parse some signatures, not sure why

Signed-off-by: Richard Zak [email protected]

[rjzak]

Working with Wasi-Crypto via --features wasi-crypto:

• Hashing SHA-256 & SHA-384
• Validating signatures against public keys for Elliptic Curves P256 & P384.
• Reading in RSA keys
• SGX attestation works

Not working:

• RSA signature parsing
• SNP attestation (needs RSA)

Not started:

• Certificate signing, since that requires reworking how Steward holds keys. Hopefully the above items provide a decent performance boost at least.