Set user agent on requests to builder (#4199)

## Issue Addressed

Closes #4185

## Proposed Changes

- Set user agent to `Lighthouse/vX.Y.Z-<commit hash>` by default
- Allow tweaking user agent via `--builder-user-agent "agent"`

beacon_node/builder_client/Cargo.toml

@@ -10,3 +10,4 @@ sensitive_url = { path = "../../common/sensitive_url" }
 eth2 = { path = "../../common/eth2" }
 serde = { version = "1.0.116", features = ["derive"] }
 serde_json = "1.0.58"
+lighthouse_version = { path = "../../common/lighthouse_version" }

beacon_node/builder_client/src/lib.rs

@@ -17,6 +17,9 @@ pub const DEFAULT_TIMEOUT_MILLIS: u64 = 15000;
 /// This timeout is in accordance with v0.2.0 of the [builder specs](https://github.com/flashbots/mev-boost/pull/20).
 pub const DEFAULT_GET_HEADER_TIMEOUT_MILLIS: u64 = 1000;
 
+/// Default user agent for HTTP requests.
+pub const DEFAULT_USER_AGENT: &str = lighthouse_version::VERSION;
+
 #[derive(Clone)]
 pub struct Timeouts {
     get_header: Duration,
@@ -41,23 +44,23 @@ pub struct BuilderHttpClient {
     client: reqwest::Client,
     server: SensitiveUrl,
     timeouts: Timeouts,
+    user_agent: String,
 }
 
 impl BuilderHttpClient {
-    pub fn new(server: SensitiveUrl) -> Result<Self, Error> {
+    pub fn new(server: SensitiveUrl, user_agent: Option<String>) -> Result<Self, Error> {
+        let user_agent = user_agent.unwrap_or(DEFAULT_USER_AGENT.to_string());
+        let client = reqwest::Client::builder().user_agent(&user_agent).build()?;
         Ok(Self {
-            client: reqwest::Client::new(),
+            client,
             server,
             timeouts: Timeouts::default(),
+            user_agent,
         })
     }
 
-    pub fn new_with_timeouts(server: SensitiveUrl, timeouts: Timeouts) -> Result<Self, Error> {
-        Ok(Self {
-            client: reqwest::Client::new(),
-            server,
-            timeouts,
-        })
+    pub fn get_user_agent(&self) -> &str {
+        &self.user_agent
     }
 
     async fn get_with_timeout<T: DeserializeOwned, U: IntoUrl>(