feat: add FreeHeadersInterval parameter to Ethereum client config (#279)

## Add FreeHeadersInterval parameter to Ethereum client config

Configure parameter `FreeHeadersInterval` set to `32` (1 epoch = 6.4
minutes) across `mainnet`, `stagenet`, and `testnet` configurations.

### Rationale
1. Aligns with Ethereum's epoch change, so it's easier to identify in
which epoch we are in
2. It's the value used in Snowbridge's test configuration

The value can be changed via pallet parameters.

---------

Co-authored-by: Steve Degosserie <723552+stiiifff@users.noreply.github.com>

operator/runtime/mainnet/src/configs/mod.rs

@@ -1190,10 +1190,14 @@ parameter_types! {
     };
 }
 
+parameter_types! {
+    pub const FreeHeadersInterval: u32 = 32; // 1 epoch = 6.4 minutes
+}
+
 impl snowbridge_pallet_ethereum_client::Config for Runtime {
     type RuntimeEvent = RuntimeEvent;
     type ForkVersions = ChainForkVersions;
-    type FreeHeadersInterval = ();
+    type FreeHeadersInterval = FreeHeadersInterval;
     type WeightInfo = mainnet_weights::snowbridge_pallet_ethereum_client::WeightInfo<Runtime>;
 }
 

operator/runtime/stagenet/src/configs/mod.rs

@@ -1189,10 +1189,14 @@ parameter_types! {
     };
 }
 
+parameter_types! {
+    pub const FreeHeadersInterval: u32 = 32; // 1 epoch = 6.4 minutes
+}
+
 impl snowbridge_pallet_ethereum_client::Config for Runtime {
     type RuntimeEvent = RuntimeEvent;
     type ForkVersions = ChainForkVersions;
-    type FreeHeadersInterval = ();
+    type FreeHeadersInterval = FreeHeadersInterval;
     type WeightInfo = stagenet_weights::snowbridge_pallet_ethereum_client::WeightInfo<Runtime>;
 }
 

operator/runtime/testnet/src/configs/mod.rs

@@ -1190,10 +1190,14 @@ parameter_types! {
     };
 }
 
+parameter_types! {
+    pub const FreeHeadersInterval: u32 = 32; // 1 epoch = 6.4 minutes
+}
+
 impl snowbridge_pallet_ethereum_client::Config for Runtime {
     type RuntimeEvent = RuntimeEvent;
     type ForkVersions = ChainForkVersions;
-    type FreeHeadersInterval = ();
+    type FreeHeadersInterval = FreeHeadersInterval;
     type WeightInfo = testnet_weights::snowbridge_pallet_ethereum_client::WeightInfo<Runtime>;
 }