refactor: re-expose scheme from client (#541)

3eb745e made many client fields
private. It exposed methods to get the port and host, but not the
scheme. We are depending on the ability to fetch the scheme in
vectordotdev/vector and so are currently blocked from upgrading.

Signed-off-by: Jesse Szwedko <jesse.szwedko@datadoghq.com>

core/src/client.rs

@@ -241,6 +241,10 @@ impl APIClient {
         self.port
     }
 
+    pub fn scheme(&self) -> &str {
+        self.scheme.as_str()
+    }
+
     async fn build_client(&mut self, name: Option<String>) -> Result<()> {
         let ua = match name {
             Some(n) => n,