feat(jans): OpenID Client and Users CustomAttribute should return appropriate attribute type

[pujavs]

Currently CustomAttributes for OpenID Client and Users return type as String.
Need changes to return appropriate CustomAttribute type

This would involve change in various modules including config-api

[moabu]

Created from a chat thread internally

[pujavs]

@yuriyz and @yurem, regarding replacing CustomAttribute with CustomObjectAttribute in User and Client models,

1. User: User class already is using CustomObjectAttribute. Refer https://github.com/JanssenProject/jans/blob/main/jans-orm/model/src/main/java/io/jans/orm/model/base/SimpleUser.java#L43

2. Client: However we will need to change Client model. Refer https://github.com/JanssenProject/jans/blob/main/jans-auth-server/common/src/main/java/io/jans/as/common/model/registration/Client.java#L193

[yuriyz]

I will change Client on AS side to use CustomObjectAttribute.

[yuriyz]

Changes on AS side are done in #2510