Update to allow use of webpki-roots where rustls-native-certs fails.

[ZaneHannanAU]

This adds the option to use webpki-roots in the cases where rustls-native-certs fails or is impossible, for example aarch64-linux-android (rustls/rustls-native-certs#3 is not possible, as the keystore is unavailable as per https://developer.android.com/training/articles/keystore), aarch64-unknown-none (it literally doesn't exist), microcontroller classes, x86_64-unknown-cloudabi/aarch64-unknown-cloudabi (also doesn't exist), wasm32-wasi (needs embedded or local thing), etc.

This additionally adds a compile time check for handling root store without required feature.

[ZaneHannanAU]

Okay, for the time being leaving out the all-features variant with regards to this:

#[cfg(all(
    feature = "tokio-runtime",
    any(not(feature = "rustls-native-certs"), feature = "webpki-roots"),
    any(not(feature = "webpki-roots"), feature = "rustls-native-certs")
))]
compile_error!(
    "Must enable exactly one of rustls-native-certs (default) or webpki-roots with tokio-runtime! (note: use `default-features = false' in a binary crate for one or other)"
);

[ZaneHannanAU]

As an alternative, I have https://github.com/ZaneHannanAU/rustls-native-certs/tree/allow-webpki-roots-on-unsupported-platform set up.

[path.crates-io]
rustls-native-certs = { git = "https://github.com/ZaneHannanAU/rustls-native-certs", branch = "allow-webpki-roots-on-unsupported-platform" }

[DoumanAsh]

To be honest considering that rustls-native-certs is not really working wel(outside of Linux)l it would be better if webpki-roots would be default option

[ctz]

Thanks!