Add a new identifier in Agents Modul

[nadinetamm]

Initial agreement (10-19-2022):

• Editing of the vocabulary yml file
• Identifier no longer as Java Enum

New agreement (11-02-2022):

• Add Managed Attributes for the agents module

[cgendreau]

I changed the agent OpenAPI specifications to have identifiers as a real resource that a person can have multiple of.
DINA-Web/agent-specs@3e81f19

The changes in agent-api implementation are following.

You can still use managed attributes for your use case, my main goal here is more to make identifiers more flexible for the future and since it's a breaking change I prefer to do it sooner than later.