feat: add is_proposer from the lean spec

[shariqnaiyer]

What was wrong?

Implement is_proposer based on the proposed validator spec.

https://github.com/unnawut/leanSpec/blob/pqdevnet0-md/docs/client/validator.md

How was it fixed?

Translated the function to rust

To-Do

• I have read CONTRIBUTING.md.
• This PR title follows conventional commits.

[Kayden-ML]

Looks good 👍

[syjn99]

@shariqnaiyer Thank you for your work!

ream/crates/common/validator/lean/src/service.rs

Lines 142 to 150 in 74dfd72

	/// Determine if one of the keystores is the proposer for the current slot.
	fn is_proposer(&self) -> Option<&LeanKeystore> {
	let current_slot = get_current_slot();
	let proposer_index: u64 = current_slot % lean_network_spec().num_validators;
	self.keystores
	.iter()
	.find(|keystore| keystore.id == proposer_index as u64)
	}

I think this work is duplicated of this function. I think we may deduplicate those functions.