feat(all): add license to project

[d1onys1us]

Issue description

To quote from https://choosealicense.com/no-permission/:

If you find software that doesn’t have a license, that generally means you have no permission from the creators of the software to use, modify, or share the software. Although a code host such as GitHub may allow you to view and fork the code, this does not imply that you are permitted to use, modify, or share the software for any purpose.

Proposed solution

1. Remove the GPL library https://github.com/privacy-scaling-explorations/zkevm-circuits/blob/main/integration-tests/contracts/greeter/Greeter.sol
2. Add a LICENSE to ~/LICENSE.md (ie. MIT, The Unlicense, GPL, CC0)

[ed255]

Currently we indicate the license of our crates in the Cargo.toml file. See https://github.com/privacy-scaling-explorations/zkevm-circuits/blob/7cd18d19453447bee58c8b774a34c62f579e3651/zkevm-circuits/Cargo.toml#L6

Our chosen license is "MIT OR Apache-2.0". I agree that we can add the license documents to the root of our repository following the approach of the halo2 repository at https://github.com/zcash/halo2/ (that is, a file named LICENSE-MIT and a file named LICENSE-APACHE). This will make github index the license information of this repository. Also we should add the following section to the README:

# License

Licensed under either of

- Apache License, Version 2.0, ([LICENSE-APACHE](https://github.com/zcash/halo2/blob/main/LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
- MIT license ([LICENSE-MIT](https://github.com/zcash/halo2/blob/main/LICENSE-MIT) or http://opensource.org/licenses/MIT)

at your option.

I also think we can expand this issue to also update the authors field in all Cargo.toml fields and set something like:

authors = ["The zkEVM Community Edition contributors"]