Keys in feerates mapping should be `u16`, not `String`.

[darosior]

In get_fee_estimates the confirmation targets are passed as strings:

rust-esplora-client/src/blocking.rs

Lines 344 to 365 in ef1925e

	/// Get an map where the key is the confirmation target (in number of blocks)
	/// and the value is the estimated feerate (in sat/vB).
	pub fn get_fee_estimates(&self) -> Result<HashMap<String, f64>, Error> {
	let resp = self
	.agent
	.get(&format!("{}/fee-estimates", self.url,))
	.call();
	let map = match resp {
	Ok(resp) => {
	let map: HashMap<String, f64> = resp.into_json()?;
	Ok(map)
	}
	Err(ureq::Error::Status(code, resp)) => Err(Error::HttpResponse {
	status: code,
	message: resp.into_string()?,
	}),
	Err(e) => Err(Error::Ureq(e)),
	}?;
	Ok(map)
	}

But they are given as u16 by electrs: https://github.com/mempool/electrs/blob/b379d24cb1972976d4b787d6e4c0f664f144bfa6/src/new_index/query.rs#L196-L205 (mempool's fork), https://github.com/Blockstream/electrs/blob/fd35014283c7d3a7a85c77b9fd647c9f09de12c9/src/new_index/query.rs#L184-L193 (Blocktream's fork).