fix: Don't leak evm specific code to interfaces(PrivateKeySigner, Chain)

Pass the signer's private key as a string around and only create a PrivateKeySigner inside Permit2 (where it's needed)
Use tycho-core Chain instead of Alloy ChainID. Create a temp method to convert between them
Create a EVMSwapEncoder that implements the executor_selector (that is evm specific)

--- don't change below this line ---
ENG-4081 Took 1 hour 7 minutes

src/encoding/swap_encoder.rs

@@ -1,6 +1,3 @@
-use alloy_primitives::FixedBytes;
-use tycho_core::keccak256;
-
 use crate::encoding::{
     errors::EncodingError,
     models::{EncodingContext, Swap},
@@ -17,9 +14,4 @@ pub trait SwapEncoder: Sync + Send {
         encoding_context: EncodingContext,
     ) -> Result<Vec<u8>, EncodingError>;
     fn executor_address(&self) -> &str;
-
-    fn executor_selector(&self) -> FixedBytes<4> {
-        let hash = keccak256("swap(uint256,bytes)".as_bytes());
-        FixedBytes::<4>::from([hash[0], hash[1], hash[2], hash[3]])
-    }
 }