Separate Execution Gas Price and DA Gas Price into separate newtypes to add clarity

[MitchTurner]

It's been suggested that we separate the two just to help organize the code a bit better. Instead of just dealing withu64 everywhere, it could help the reader to have types to communicate everywhere in the code the purpose of the numbers.

[feifies]

您好，我想要修复 Issue #2382，但对项目的代码结构还不太熟悉，能否提供相关的文件名或模块的指引？非常感谢！

[AurelienFT]

@feifies Thanks for your interest and sorry for late reply. The goal is to change the type of all exec_gas_price such as here : https://github.com/FuelLabs/fuel-core/blob/e1ff8d47ceaeea84f24f1647fd47eebe297bbcf9/crates/services/gas_price_service/src/v1/metadata.rs#L64 from u64 to a new type type ExecGasPrice = u64 and same for all da_gas_price to something like type DAGasPrice = u64