Add check-files-licence-headers job

[Jonas-Sander]

The check-files-licence-headers job checks that all files contain the appropriate licence header:

# Copyright (c) 2022 Sharezone UG (haftungsbeschränkt)
# Licensed under the EUPL-1.2-or-later.
#
# You may obtain a copy of the Licence at:
# https://joinup.ec.europa.eu/software/page/eupl
#
# SPDX-License-Identifier: EUPL-1.2

which has its template definied under header_template.txt.

If one wants to automatically add the licence headers then the command

addlicense -check -c "Sharezone UG (haftungsbeschränkt)" -f header_template.txt .

can be run.
The tool has to be installed via go install github.com/google/addlicense@latest first.

See: https://github.com/google/addlicense

Closes #25

[nilsreichardt]

LGTM