Swagger docs: Add format info for date attributes with type integer

[JuAlMan]

This issue is referring to date attributes like MetaData.credential_valid_from etc.
Those attributes store unix epoch timestamps and are therefore of type integer.

Unfortunately our code generation tools for Java generate Java Integer type for those attributes which can only store 32-bit-integers.
We would therefore appreciate if you could add an additional format info specifying int64 for those attributes like this:

type: integer
format: int64

[masv3971]

Does this solve the problem? a2d0921