client/server: crypto_provider accessor for configs

When holding a `ClientConfig` or a `ServerConfig` it may be helpful to
be able to access the `&Arc<CryptoProviver>` that will be used for the
configuration. This commit adds accessor functions for this purpose.

rustls/src/client/client_conn.rs

@@ -325,6 +325,11 @@ impl ClientConfig {
         }
     }
 
+    /// Return the crypto provider used to construct this client configuration.
+    pub fn crypto_provider(&self) -> &Arc<CryptoProvider> {
+        &self.provider
+    }
+
     /// Access configuration options whose use is dangerous and requires
     /// extra care.
     pub fn dangerous(&mut self) -> danger::DangerousClientConfig<'_> {

rustls/src/server/server_conn.rs

@@ -469,6 +469,11 @@ impl ServerConfig {
         }
     }
 
+    /// Return the crypto provider used to construct this client configuration.
+    pub fn crypto_provider(&self) -> &Arc<CryptoProvider> {
+        &self.provider
+    }
+
     /// We support a given TLS version if it's quoted in the configured
     /// versions *and* at least one ciphersuite for this version is
     /// also configured.