Ensure `tlsn-core` and `tlsn-attestation` can compile on unknown targets

[sinui0]

Both tlsn-core and tlsn-attestation should be able to compile without depending on syscalls (e.g. I/O, RNG).

[themighty1]

As of now, we don't use getrandom or any other syscalls in the presentation verification path.

The solution for unknown targets is to disable the getrandom backend with rustflags = ['--cfg', 'getrandom_backend="unsupported"']

On tlsn side, we can add a feature like e.g.

  # crates/core/Cargo.toml or crates/attestation/Cargo.toml
  [features]
  no-syscall-verification = []  # Marker feature, requires RUSTFLAGS config

  Then in docs/README:

  no-syscall-verification feature

  For environments without syscall support, enable this feature AND set:
  RUSTFLAGS='--cfg getrandom_backend="unsupported"'
  This works for any target architecture.

I made an example which proves that this approach works https://github.com/tlsnotary/tlsn/tree/4d35f66f3edd36f7378adf243ea24c230e0dd203/crates/attestation/examples