[UK Toolkit] Add missing getters to UKRegistrationRequest

[imesh94]

Currently there is an incorrect behavior in DCR POST mandatory parameter validation. When some dcr registration request parameters are marked as 'required' in deployment.toml, the validation rejects the request even if those parameters are present. This is due to incorrect object mapping at [1], when the validation is called by the toolkit. Incorrect mapping happens due to the missing getters in the UKRegistrationRequest object.

ob-internal: https://github.com/wso2-enterprise/wso2-ob-internal/issues/1252

[1] - https://github.com/wso2/financial-services-accelerator/blob/3.0.0/open-banking-accelerator/components/com.wso2.openbanking.accelerator.identity/src/main/java/com/wso2/openbanking/accelerator/identity/dcr/validation/RequiredParamsValidator.java#L52