Include HtsSystemContract.json in version control

[acuarica]

Most of the issues included in the #146 epic, use HTS forking from Hardhat. However, currently Hardhat users are unable to use our solution until we publish the package to npm #54. But at the same time, it would be good to have user validation before publishing.

At the start of this project, we try to solve this issue by including HtsSystemContract.json in the repo so that Hardhat users can consume the plugin directly from GitHub.

We removed it #122 when we had multiple npm packages.

Given that after #139 we have a single npm package again, we can re-include the .json output so that Hardhat users can consume the repo without the package being published yet.

Once we have a strong validation from users, we can publish to npm as originally planned.