chore(gas_price_service): add a method to wait until storages are synced

[rymnc]

since we have the gas price service depending upon the onchain database as well as the gas price database, we need a reliable way to run tests such that the gas price database is known to have been updated by some state change in the onchain database i.e new L2 block, for example.

#2874 (comment)

the relayer provides similar functionality via await_synced:

fuel-core/crates/services/relayer/src/service.rs

Lines 298 to 309 in 90c855c

	pub async fn await_synced(&self) -> anyhow::Result<()> {
	let mut rx = self.synced.clone();
	loop {
	if rx.borrow_and_update().is_synced() {
	break;
	}
	rx.changed().await?;
	}
	Ok(())
	}

Definition of done

• the gas price service updates the status when it receives a new block to NotSynced and when it is done processing the block, updates it to Synced(height)
• The tests(unit + integ) are updated to remove sleep's and instead use the new await_synced method with a timeout instead

[rymnc]

you can use 0a2ea74 as reference on how to do it