Strongly typed API for asynchronous TDLib method execution in Java interface

[vkryl]

This PR includes:

• Strongly typed Client.send methods, callback parameter type of which is based on the return type declared in TdApi.Function child class, which allow compile-time protection against return type changes, avoiding casts on caller side, etc. Weakly typed send methods remain unchanged for compatibility purposes.
• Client.send method without any callbacks. The only reason for it is because client.send(function, null) will now cause compile error
• Warning fix for raw parametrized use of TdApi.Function
• Small documentation fix in Client.java

This allows using the following pattern when executing methods in Java:

client.send(new TdApi.SpecificReturnTypeFunction(), (typedResult, error) -> {
  if (error != null) {
    // handle error
  } else {
    // work with typedResult without any casting
  }
});