Optional rustls support

[seanmonstar]

With the ability to now optionally disable the default-tls feature (#375), it would also be fantastic to allow people to use reqwest with a rustls backend instead.

• We can add rustls as a dependency, optional = true. This will automatically promote to a rustls feature, defaulted off.
• The code inside the connect module can likely inline the connector code from hyper-tls and hyper-rustls, so we can remove that dependency.
• The transport should just be a trait object, essentially Box<AsyncRead + AsyncWrite>.
• If both default-tls and rustls are enabled, the native-tls backend should be chosen by default.
• The ClientBuilder can gain methods to pick a backend at runtime.

[timleg002]

Shouldn't rustls be chosen by default when both are enabled? To avoid verboseness via default-features = false

[evbo]

For anyone else wondering how to add rust-tls, set feature flags like the following in your Cargo.toml:

reqwest = { 
    version = "0.11.16", 
    features = ["json", "rustls-tls"], 
    default-features = false 
}

[xpe]

Using features = ["json", "rustls-tls"], default-features = false works, but, however on my MacBook Pro M2 at least, trace-level logging looks awful. (It worked fine on the default settings, FWIW).

e.g. (using env_logger):

[2024-01-11T21:50:08Z DEBUG reqwest::connect] starting new connection: https://api.foobarbuzzapp.website/
[2024-01-11T21:50:08Z TRACE reqwest::connect::verbose] 0897ca88 write: b"PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n"
[2024-01-11T21:50:08Z TRACE reqwest::connect::verbose] 0897ca88 write: b"\0\0\x12\x04\0\0\0\0\0\0\x02\0\0\0\0\0\x04\0 \0\0\0\x05\0\0@\0\0\0\x04\x08\0\0\0\0\0\0O\0\x01"

UPDATE: Ugly logging, yes, but this is good -- it is HTTP 2/at work. It is good that rust-tls made it happen :)