refactor: move signing data (source, signature) directly to InjectedTransaction

### What is suboptimal now?

ethexe/common/src/injected.rs

### Proposed change

Current 
```rust
pub type SignedInjectedTransaction = SignedMessage<InjectedTransaction>;
``` 
must be restructured. 

`InjectedTransaction` must store source and transaction signature.

### Impact

_No response_