add request header override option

src/proxy/backend_opt.rs

@@ -0,0 +1,16 @@
+use crate::error::*;
+
+#[derive(Debug, Clone, Hash, Eq, PartialEq)]
+pub enum UpstreamOption {
+  OverrideHost,
+  // TODO: Adds more options for heder override
+}
+impl TryFrom<&str> for UpstreamOption {
+  type Error = anyhow::Error;
+  fn try_from(val: &str) -> Result<Self> {
+    match val {
+      "override_host" => Ok(Self::OverrideHost),
+      _ => Err(anyhow!("Unsupported header option")),
+    }
+  }
+}