add ability to create a client with own tls connector (#809)

src/blocking/client.rs

@@ -1,3 +1,4 @@
+use std::any::Any;
 use std::convert::TryInto;
 use std::fmt;
 use std::future::Future;
@@ -450,6 +451,15 @@ impl ClientBuilder {
         self.with_inner(move |inner| inner.use_rustls_tls())
     }
 
+    /// Use a preconfigured TLS backend.
+    ///
+    /// If the passed `Any` argument is not a TLS backend that reqwest
+    /// understands, the `ClientBuilder` will error when calling `build`.
+    #[cfg(feature = "__tls")]
+    pub fn use_preconfigured_tls(self, tls: impl Any) -> ClientBuilder {
+        self.with_inner(move |inner| inner.use_preconfigured_tls(tls))
+    }
+
     // private
 
     fn with_inner<F>(mut self, func: F) -> ClientBuilder