Adding Mint Token operation

[rbaconcordium]

Purpose

To add code enabling minting of a Protocol Level Token

Changes

MintTokenOperation class added to enable minting

Checklist

• [ x ] My code follows the style of this project.
• [ x ] The code compiles without warnings.
• [ x ] I have performed a self-review of the changes.
• [ x ] I have documented my code, in particular the intent of the
  hard-to-understand areas.
• [ x ] (If necessary) I have updated the CHANGELOG.

CLA acceptance

_Remove if not applicable.

By submitting the contribution I accept the terms and conditions of the
Contributor License Agreement v1.0

• link: https://developers.concordium.com/CLAs/Contributor-License-Agreement-v1.0.pdf

• I accept the above linked CLA.

[Radiokot]

Richard, please change the version to 12.1.0-alpha.2 and in the Changelog, put both your lines and the lines from "12.1.0" under the "Unreleased" secrion. It was my mistake to include "12.1.0" to the Changelog, this version is not yet released