Add SSZ types, content negotiation, and encoding helpers

Introduce EncodingType, AcceptedEncodings, and content negotiation
primitives for SSZ support in the PBS pipeline. Includes:

- EncodingType enum with FromStr/Display and MIME param tolerance
- AcceptedEncodings with q-value aware Accept header parsing
- SSZ bid value extraction from SignedBuilderBid by fork
- deserialize_body and parse_response_encoding_and_fork helpers
- Per-fork type aliases for BuilderBid and ExecutionPayloadHeader
- SszValueError, PbsError::GeneralRequest, PbsClientError variants
- Comprehensive unit tests for all new types and helpers
- New deps: headers-accept, mediatype

Author: JasonVranek

crates/common/src/config/mux.rs

@@ -236,7 +236,6 @@ impl MuxKeysLoader {
                         "Mux keys URL {url} is insecure; consider using HTTPS if possible instead"
                     );
                 }
-                let url = url.as_str();
                 let client = reqwest::ClientBuilder::new().timeout(http_timeout).build()?;
                 let response = client.get(url).send().await?;
                 let pubkey_bytes = safe_read_http_response(response, MUXER_HTTP_MAX_LENGTH).await?;

crates/pbs/src/mev_boost/register_validator.rs

@@ -187,8 +187,8 @@ async fn send_register_validator(
         .with_label_values(&[code.as_str(), REGISTER_VALIDATOR_ENDPOINT_TAG, &relay.id])
         .inc();
 
-    safe_read_http_response(res, MAX_SIZE_DEFAULT).await.inspect_err(|e| {
-        error!(relay_id = relay.id.as_ref(), retry, %e, "failed registration");
+    safe_read_http_response(res, MAX_SIZE_DEFAULT).await.inspect_err(|err| {
+        error!(relay_id = relay.id.as_ref(), retry, %err, "failed registration");
     })?;
 
     debug!(

crates/pbs/src/mev_boost/status.rs

@@ -73,8 +73,8 @@ async fn send_relay_check(relay: &RelayClient, headers: HeaderMap) -> Result<(),
     let code = res.status();
     RELAY_STATUS_CODE.with_label_values(&[code.as_str(), STATUS_ENDPOINT_TAG, &relay.id]).inc();
 
-    safe_read_http_response(res, MAX_SIZE_DEFAULT).await.inspect_err(|e| {
-        error!(relay_id = relay.id.as_ref(), %e, "status failed");
+    safe_read_http_response(res, MAX_SIZE_DEFAULT).await.inspect_err(|err| {
+        error!(relay_id = relay.id.as_ref(), %err, "status failed");
     })?;
 
     debug!(relay_id = relay.id.as_ref(),?code, latency = ?request_latency, "status passed");

crates/pbs/src/mev_boost/submit_block.rs

@@ -203,8 +203,8 @@ async fn send_submit_block(
 
     let response_bytes = safe_read_http_response(res, MAX_SIZE_SUBMIT_BLOCK_RESPONSE)
         .await
-        .inspect_err(|e| {
-            warn!(relay_id = relay.id.as_ref(), retry, %e, "failed to get payload (this might be ok if other relays have it)");
+        .inspect_err(|err| {
+            warn!(relay_id = relay.id.as_ref(), retry, %err, "failed to get payload (this might be ok if other relays have it)");
         })?;
 
     if api_version != &BuilderApiVersion::V1 {