refactor(async): add common GET and POST methods

- remove duplicated HTTP client code for handling GET and POST requests.
- adds a few new methods to `AsyncClient` implementation, the new
  methods are responsible to handle common HTTP requests and parsing. It
  was previously duplicated throughout the Esplora API implementation,
  but now it follows the same approach already implemented for
  blocking client (`BlockingClient`).

Author: oleonardolima