Submission: [serenity] Update field type on Challenge Service #30047223 - (bigint NOT NULL)

Assumptions:
* Leave field names as createdBy and updatedBy
* createdBy and updatedBy must not be null (bigint NOT NULL)

Updated field type to bigint in schemas
Updated field type in models 
Updated swagger file

Following tests should be rewritten or deleted (now disabled):
Challenges Controller - Challenges API - Paging, orderBy and filtering in get all challenges API
* should able to get nulls first by nulls first in the challenges response
* should able to get nulls last by nulls last in the challenges response
* should fail to get challenges without first or last in nulls filter