Assign and display governance sensitivity labels

[ChristophWurst]

Is your feature request related to a problem? Please describe.

As a Nextcloud Mail user, I want to tag my outgoing emails with sensitivity labels, and see when an incoming email was tagged with one.

Describe the solution you'd like

Let the user select from a list of sensitivity labels of the enterprise governance app. Write the label as X-NC-Governance-Label header with id=<snowflake>; origin=<instance-id>

Work packages

• Sending: fetch list of sensitivity label, show dropdown, write header
• Receiving: read header in backend, show it in frontend on top of the open email

Out of scope

• Editing of sensitivity labels for data at rest (email in a mailbox)

Describe alternatives you've considered

• IMAP permflags -> do not survive sending, user could remove them
• Standard headers -> do not fit the use case

Additional context

• Related: Certian Mail Labels should require S/MIME Encryption #11136
• Sensitivity: (RFC 2156/4021; Personal/Private/Company-Confidential) labels
• SIO-Label: (RFC 7444)