Add validator to all belgif openapi builds

[pvdbosch]

Now that the validator is available as open source, we should add it to the build of each belgif openapi project. edit: Also configure dependabot to keep version up to date

[pvdbosch]

done for openapi-healthcare, to do for the other ones

[jpraet]

Probably also best to setup dependabot then, so the validator plugin version is kept up-to-date.

[pvdbosch]

will be done at same time as #173