Dual license as Apache2 + GPL3

[jsdw]

Currently subxt is under the GPL3 license.

We've had a chat internally and are open to moving to dual licensing this code under Apache2 + GPL3 (Users to have the option to decide which license they apply; Apache2 to use the lib fairly freely or GPL3 if they'd like to fork it and keep the open source requirement on further forks).

Let's:

• update the license files
• update the README to explain
• update the file headers (I'm not sure what is most appropriate here for a dual license offhand; let's check!)

[jsdw]

I think that dual licensing could involve doing the following:

Update LICENSE to be something like:

Copyright 2019-2022 Parity Technologies (UK) Ltd.

This program is free software: you can redistribute it and/or modify
it under the terms of (at your option) either the Apache License,
Version 2.0, or the GNU General Public License as published by the
Free Software Foundation, either version 3 of the License, or (at your
option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

For details and specific language governing permissions and
limitations, see either

 - http://www.gnu.org/licenses/ for the GNU GPL
 - http://www.apache.org/licenses/LICENSE-2.0 for the Apache license

And then at the top of each file, add something like:

// Copyright 2019-2022 Parity Technologies (UK) Ltd.
// This file is dual-licensed as Apache 2.0 and GPLv3.
// see LICENSE for license details.

This, at least, is the best I could find from a bit of a google on what others suggest/have done.

Any thoughts?