use `EngineApi` from `alloy`

I feel the methods in this file are obsolete

https://github.com/informalsystems/emerald/blob/main/engine/src/engine_rpc.rs

because `alloy-provider` already offers them under `engine-api` feature.

https://docs.rs/alloy-provider/latest/alloy_provider/ext/trait.EngineApi.html

Start with engine/src/engine_rpc.rs and compare its methods with alloy-provider's EngineApi under the engine-api feature, using the linked documentation as the reference. Confirm which local methods are duplicated, replace their usage with the alloy API where appropriate, and verify that the engine RPC functionality remains covered by the repository's existing checks.

Written by the indexing model from the issue text.