Fetch BTC/USD price with http request of offchain worker

[mzxyz]

1. Trying to use Http module in substrate/core/offchain

This module is composed of two structs: [`HttpApi`] and [`HttpWorker`]. Calling the [`http`]
function returns a pair of [`HttpApi`] and [`HttpWorker`] that share some state.

The [`HttpApi`] is (indirectly) passed to the runtime when calling an offchain worker, while
the [`HttpWorker`] must be processed in the background. The [`HttpApi`] mimicks the API of the
HTTP-related methods available to offchain workers.

The reason for this design is driven by the fact that HTTP requests should continue running
(i.e.: the socket should continue being processed) in the background even if the runtime isn't
actively calling any function.

2. Reference for requesting code

3. Using coinmarketcap api to request BTC/USD price, JS code sample

const rp = require('request-promise');
const requestOptions = {
  method: 'GET',
  uri: 'https://pro-api.coinmarketcap.com/v1/cryptocurrency/quotes/latest?id=1',
  headers: {
    'X-CMC_PRO_API_KEY': '*****'
  },
  json: true,
  gzip: true
};

rp(requestOptions).then(response => {
  console.log('API call response:', response.data[1].quote.USD.price);
}).catch((err) => {
  console.log('API call error:', err.message);
});

4. Temporary solution

 let uri = "https://pro-api.coinmarketcap.com/v1/cryptocurrency/quotes/latest?id=1";
        let api_key_value = "*****";
        let api_key = "X-CMC_PRO_API_KEY";

        let id: HttpRequestId = runtime_io::http_request_start("GET", uri, &[0]).unwrap();
	    match runtime_io::http_request_add_header(id, api_key, api_key_value) {
		    Err(_) => { debug("Add request header failed !!!") }
		    Ok(_) => {  debug("Add request header succeed") }
	    };

        let deadline = runtime_io::timestamp().add(Duration::from_millis(10_000));
        match runtime_io::http_response_wait(&[id], Some(deadline))[0] {
			HttpRequestStatus::Finished(200) => {debug("request succeed")},
			_ => {debug("request failed")}
		}

        let buffer_len = 2048;
        let mut buf = Vec::with_capacity(buffer_len as usize);
		buf.resize(buffer_len as usize, 0);
		let res = runtime_io::http_response_read_body(id, &mut buf, Some(deadline));
        match res {
            Ok(read) => { runtime_io::print_utf8(&buf[..read])}
            Err(_) => {debug("parse body failed")}
        }