Fix for populating fields via idp

[S203932]

I changed the name of the field within the front-end to from registerCertificate to certificate. Thus, being identically within the one defined in the back-end.

[sonarqubecloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
92.5% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[S203932]

What's being fixed with this pull request:

1. populating the registrations fields with the Idp information
2. OIDC and SAML can now also use this feature (the X509 filter wasn't present when doing Idp registration and it didn't populate the necessary session attributes)
3. Error's from the certificate expiration pop-up window are now being caught
4. The certificate field can be omitted from the application-registration.yml (along with all the other fields)

One thing to take note off is that the certificate field is part of the registration fields and therefore inherits all the properties of RegistrationFieldProperties. This means that one can configure readOnly and fieldBehaviour for certificate, though there is no benefit from this. Also, it should be noted that one should not set readOnly = true for certificate since this means that it is the Idp to provide the information about whether the certificate should be linked or not. If one does set this value to true, then it would most likely result in the user not being able to register as the Idp is not providing this required information for the registration.

Please advise, if any changes are needed.