feat: allow clients to override client_name (#469)

Many MCP Servers use client_name for a variety of things including:
* Whitelisting
* Logos
* Copy shown directly on the page
* etc

As a result, it's important for MCP Clients to be able to override the client name.

examples/clients/src/auth/oauth_client.rs

@@ -100,7 +100,11 @@ async fn main() -> Result<()> {
         .await
         .context("Failed to initialize oauth state machine")?;
     oauth_state
-        .start_authorization(&["mcp", "profile", "email"], MCP_REDIRECT_URI)
+        .start_authorization(
+            &["mcp", "profile", "email"],
+            MCP_REDIRECT_URI,
+            Some("Test MCP Client"),
+        )
         .await
         .context("Failed to start authorization")?;