Context:
In the context of allowing to define to which definition a particular report filter applies it is required to introduce and identifier to definition (in ui data source) entries.
See https://confluence.camunda.com/display/CO/2021-06-09+Multi-Definition+Filters%3A+Technical+Kickoff

AT:

• the definitions dto is extended with a `identifier` property
• the identifier field is required to be non-empty on any evaluate/update request, requests lacking a value are rejected
• the identifier field is added during the migration and contains a random uuid
• rest api docs definitions dto are adjusted accordingly