Add license to yaml files

[aethy]

The OpenApi spec provides a "license" property in the Info Object to indicate the license type. As the belgif data types repositories already have a license (e.g. https://github.com/belgif/openapi-person-identifier/blob/master/LICENSE), maybe we should add this license in the yaml files too (for all belgif OpenAPI data types repositories).

[pvdbosch]

OK for WG to add Apache License 2.0 in all belgif openapi files.

[pvdbosch]

license added for employment-identifier (while changing it for another issue):

• https://github.com/belgif/openapi-employment-identifier/blob/master/src/main/openapi/employment/identifier/v1/employment-identifier-v1.yaml
• https://github.com/belgif/openapi-employment-identifier/blob/master/src/main/swagger/employment/identifier/v1/employment-identifier-v1.yaml

TODO for other openapi repos

[JDMKSZ]

I did it for 1 project: https://github.com/belgif/openapi-money/pull/4 If no remarks on that PR, I can do it for all the projects.