Add tag field to TaskAPI

Registered by haruka tanizawa

This is a new feature of TaskAPI.
With this specification, user can add 'tag' to Nova API when user requests.
'tag' is string or UUID.

For example, user can know task status of request API,
even if user couldn't get response from nova, on condition that user requested with 'tag'.

nova-specs comming soon...

Blueprint information

Status:
Not started
Approver:
None
Priority:
Undefined
Drafter:
haruka tanizawa
Direction:
Needs approval
Assignee:
None
Definition:
New
Series goal:
None
Implementation:
Unknown
Milestone target:
None

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