refactor: Make group_swaps a util method

- In preparation for also using this for the executor encoder.
This also make test setup much easier for swap grouping - not having to init the encoder every time.

src/encoding/evm/strategy_encoder/mod.rs

@@ -1,3 +1,4 @@
 pub mod strategy_encoder_registry;
 mod strategy_encoders;
 mod strategy_validators;
+mod utils;