* Fix rustls-tls feature
Commit 14e74879 (cookie support #1146) re-introduced an unconditional
dependency on the openssl-sys crate. That is, building Lychee with the
Rustls TLS backend now requires OpenSSL. I suppose this change was
unintended, maybe due to automatic conflict resolution. If not, please
let me know.
You can review the re-introduced dependency like so:
```
cargo tree --no-default-features --features rustls-tls -i openssl-sys
```
This commit puts the OpenSSL dependency behind the native-tls feature
flag again.
You can check the TLS features like so:
```
cargo check --workspace --all-targets --features vendored-openssl
cargo check --workspace --all-targets --all-features
cargo check --workspace --all-targets --no-default-features --features rustls-tls
```
Maybe this should be added to CI. But I don't want to waste anybody's
time.
* Check feature flags during CI
Adds a new CI job 'check-feature-flags' to verify the following:
- Lychee with rustls-tls feature only doesn't depend on OpenSSL
- Cargo check passes with default features
- Cargo check passes with all features
- Cargo check passes with rustls-tls feature only