Move Context from GSN to utils directory

[Amxx]

Proposal to move the Context contract to the contracts/utils folder

[frangio]

To provide some context: we want to move this file out of the GSN directory to make it clear that Context itself has nothing to do with GSN. It enables opting in to GSN if a developer wants to do so.

[frangio]

Thanks!

For backwards compatibility we need to keep a file contracts/GSN/Context.sol that just imports the file from the new location:

// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

import '../utils/Context.sol';