Allow "Source" and "Version" in INFO header

[maehler]

This PR adds | fixes:

The fields "Source" and "Version" are optional in the INFO header lines according to the VCF specification (v4.2 and above), but the validation does not take this into account. This PR allows for these optional fields.

Review:

• [ ] Code approved by
• [ ] Tests executed by
• [ ] "Merge and deploy" approved by

This version is a:

• [ ] MAJOR - when you make incompatible API changes
• [ ] MINOR - when you add functionality in a backwards compatible manner
• [x] PATCH - when you make backwards compatible bug fixes or documentation/instructions