Refactor genesis testing params into HiveParams struct

[Rjected]

Is your feature request related to a problem? Please describe.

Currently the Genesis struct includes parameters that are used only for testing:

ethers-rs/ethers-core/src/utils/genesis.rs

Lines 48 to 64 in 37acf65

	// The following fields are only included for tests, and should not be used in real genesis
	// blocks.
	/// The block number
	#[serde(skip_serializing_if = "Option::is_none", default)]
	pub number: Option<U64>,
	/// The block gas gasUsed
	#[serde(skip_serializing_if = "Option::is_none", default)]
	pub gas_used: Option<U64>,
	/// The block parent hash
	#[serde(skip_serializing_if = "Option::is_none", default)]
	pub parent_hash: Option<H256>,
	/// The base fee
	#[serde(skip_serializing_if = "Option::is_none", default)]
	pub base_fee_per_gas: Option<U256>,

Describe the solution you'd like
These should be refactored out into a struct in combination with serde(flatten)

Additional context

I would consider putting this in a hive: HiveParams w/ serde(flatten) but we can leave for follow-up, we don't use the genesis type in other places either way

Originally posted by @gakonst in #2145 (comment)