feat(bundler): Add pool client interface and bundler orchestration (BA-3414)

- Add PoolClient trait for fetching UserOperations from mempool
- Add Bundler struct that orchestrates pool fetching, gas tracking, and bundle building
- Add NoOpPoolClient for when AA bundling is disabled
- Update default gas threshold from 80% to 50% for middle-of-block bundles
- Support both v0.6 (unpacked) and v0.7 (packed) UserOperation formats
- Add entry point address constants

TODO: Implement RemotePoolClient that connects to tips mempool service via RPC

Author: shamit05

crates/op-rbuilder/src/args/op.rs

@@ -97,10 +97,10 @@ pub struct OpRbuilderArgs {
     )]
     pub aa_gas_reserve_percentage: u8,
 
-    /// Threshold percentage of block gas before starting AA bundle reservation
+    /// Threshold percentage of block gas before starting AA bundle creation (middle of block)
     #[arg(
         long = "aa.gas-threshold",
-        default_value = "80",
+        default_value = "30",
         env = "AA_GAS_THRESHOLD"
     )]
     pub aa_gas_threshold: u8,

crates/op-rbuilder/src/builders/mod.rs

@@ -202,7 +202,7 @@ impl<S: Default + Clone> Default for BuilderConfig<S> {
             enable_aa_bundler: false,
             aa_bundler_signer: None,
             aa_gas_reserve_percentage: 20,
-            aa_gas_threshold: 80,
+            aa_gas_threshold: 30,
             aa_pool_url: None,
         }
     }