Rewrite submit_block relay communication to support SSZ and JSON

content negotiation. Includes:
- SSZ-first request encoding with JSON fallback on 406/415
- Content-Type and Eth-Consensus-Version header handling
- Fork-aware SSZ decoding for relay responses
- MIME parameter tolerance on relay response Content-Type
- v2 to v1 fallback forwards payload to BN (prevents silent block loss)
- V2 fallback metric counter
- Comprehensive submit_block integration tests for both encodings

Author: JasonVranek

crates/pbs/src/metrics.rs

@@ -60,4 +60,14 @@ lazy_static! {
         &["http_status_code", "endpoint"],
         PBS_METRICS_REGISTRY
     ).unwrap();
+
+    /// Count of v2 submit_block requests that fell back to the v1 endpoint
+    /// because the relay returned 404 on v2. A high value indicates the relay
+    /// fleet has not been upgraded to support submitBlindedBlockV2.
+    pub static ref V2_FALLBACK_TO_V1: IntCounterVec = register_int_counter_vec_with_registry!(
+        "pbs_submit_block_v2_fallback_to_v1_total",
+        "Count of v2 submit_block requests that fell back to v1 because the relay did not support v2",
+        &["relay_id"],
+        PBS_METRICS_REGISTRY
+    ).unwrap();
 }