Cargo.toml: make rustls-platform-verifier feature as optional

[xz-dev]

#253 add the function in code, but it's no way to enable the feature, because Cargo.toml not have the feature.

[ctz]

I thought each optional dependency comes with an implicit feature of the same name?

[xz-dev]

I had checked the document, but when I want to enable it:

❯ cargo add hyper-rustls --features rustls-platform-verifier
    Updating crates.io index
      Adding hyper-rustls v0.26.0 to dependencies.
error: unrecognized feature for crate hyper-rustls: rustls-platform-verifier
disabled features:
    webpki-tokio
enabled features:
    http1, http2, log, logging, native-tokio, ring, rustls-native-certs, tls12
    webpki-roots

I'm confused, can you give me an example?

[djc]

The feature was added after the last release IIRC.