addition and updates to CI workflows

[smallTrogdor]

TL;DR

• add release workflow
• update test workflow

Longer Explanation

With this PR, there are two updates being introduced to our workflow.

New Workflow

First, a new Design System Flutter Release workflow is introduced: The workflow is manually triggered with patch, minor or major argument and turns the [Unreleased] section of the CHANGELOG.md into a tag, an automatic commit and a consequent github release fully automatic. It also updates the pubspec.yaml file in accordance.

Updated Test Workflow

Second, the test workflow is triggered on every PR to main (previously it was triggered on every push to any branch) and includes a validation of the CHANGELOG.md schema defined in Keep A Changelog. Please note that this includes the allowed sub sections in the version releases (e.g. a Removed section cannot be included in a patch or minor version release).

We aim to further go into the GitHub Flow direction - i.e. having short lived feature branches with the addition of fully controlled releases (slower).

[smallTrogdor]

I also updated the CHANGELOG.md file to have all the GitHub releases from previous releases.

[smallTrogdor]

superseded by #118