Make metadata expiration time configurable

[AramMutluWork]

The expiration time of the metadata defines the validUntil until attribute in the metadata of Engineblock. By default this a constant in the code set to 86.400 seconds (24 hours).

At the request of one of the connected Service Providers, we want to increase this value to 1,814,400 seconds (21 days). If something goes wrong with the automatic reading of our metadata, they will have more time to fix this error before users experience login problems.

To avoid having to set this up again with every release, we want to make this configurable, I left the default at 24 hours as before.

[MKodde]

A nice and useful new contribution!

I see a lot of nice things, but looking at this critically, I have some suggestion/questions:

1. Now that the exp time is no longer hard coded; would we not need some validation/verification if a correct value is configured? What happens with negative values, floating points, string values,... This validation does not need to be very comprehensive, and by looking at your type definition, the application only allows for integer values. But that can still be a negative value. Maybe check for that in the MetadataRenderer constructor?
2. I'd love to see the MetaDataRendererTest be extended with a couple of additional tests to verify that the expected expiration time matches with the one configured

[MKodde]

Aside from fixing the test by passing the new parameter, it might be very useful to add a couple of tests that verify the expiration time ends up correctly in the metadata?