cannot configure custom cert in Manage UpdateSites

### Your checklist for this issue

🚨 Please review the [guidelines for contributing](../blob/master/docs/CONTRIBUTING.md) to this repository.

- [x] Jenkins version: 2.204.2

- [x] Plugin version:
Configuration as Code: 1.35
UpdateSites Manager plugin: 2.0.0

- [x] OS: Ubuntu

### Description
We are using custom update sites (Jenkins -> Manage Jenkins -> Manage UpdateSites), that we want to maintain with CASC on our jenkins masters. Problem here is: reading the casc reference, we can only specify id and url ... but in the UI we can also check a "Need CA Certificate" checkbox and enter a "CA Certificate" there ...

So via CASC you currently can only add sites, that don't require a custom CA certificate.

Managing Update Sites comes with "UpdateSites Manager plugin", but I already asked in gitter channel, and that plugin looks okay, so we decided to raise an issue here.