rustls: do not depend on unnecessary features

The main motivation for that change is to prevent an unnecessary dependency to
rustls/aws_lc_rs [0]. aws_lc_rs has a non-default build with external tools
(cmake, nasm) which breaks CI in consumer projects, such as ttfb [1]. So instead
of forcing it, we make it optional.

[0]: https://docs.rs/rustls/latest/rustls/#crate-features
[1]: https://github.com/phip1611/ttfb/

Author: phip1611

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name          = "rustls-connector"
-version       = "0.20.0"
+version       = "0.21.0"
 authors       = ["Marc-Antoine Perennou <[email protected]>"]
 edition       = "2021"
 description   = "Connector similar to openssl or native-tls for rustls"
@@ -20,7 +20,11 @@ native-certs       = ["rustls-native-certs", "log"]
 webpki-roots-certs = ["webpki-roots"]
 
 [dependencies]
-rustls = "^0.23"
+# Caution. No default crypt provider. Users may have to add `rustls´
+# additonally as dependency and specify "ring" or "aws_lc_rs". Note that
+# although "aws_lc_rs" is the default, it doesn't build as easily on all
+# platforms. See https://docs.rs/rustls/latest/rustls/#crate-features.
+rustls = {  version = "^0.23", default-features = false, features = ["std"] }
 rustls-webpki = "^0.102"
 
 [dependencies.log]