feat: Implement SplitSwapStrategyEncoder

The strategy works as follows:
- Manage approvals needed
- Compute min amount (if check amount is any):
  - if slippage is defined, apply slippage on the expected amount and take the min value between that and the check amount
  - if not, it's just the check amount
- Iterate through the swaps
  - call the corresponding swap encoder to encode the swap
  - add swap header (tokens indexes and split)
  - ple encode the swaps
- Add extra inputs (amounts, token addresses, min amount, (un)wrap, number of tokens and receiver)

Misc:
- Move executor address and selector encoding inside the SwapEncoder
- Add default executor_selector to SwapEncoder
- Pass router address inside the SplitSwapStrategyEncoder
- Move Permit2 inside the SplitSwapStrategyEncoder. It is a responsibility and a specificity of the strategy to need permit2 approvals

--- don't change below this line ---
ENG-4081 Took 1 hour 21 minutes

src/encoding/router_encoder.rs

@@ -10,5 +10,4 @@ pub trait RouterEncoder<S: StrategySelector> {
         &self,
         solutions: Vec<Solution>,
     ) -> Result<Vec<Transaction>, EncodingError>;
-    fn handle_approvals(&self, solutions: &[Solution]) -> Result<Vec<Vec<u8>>, EncodingError>;
 }