R1:cbe4e68c0722

Add a Herald action to trigger "Must Encrypt" for mail

Summary: Depends on D18983. Ref T13053. Adds a new Herald action to activate the "must encrypt" flag and drop mail content.

Test Plan:
- Created a new Herald rule:

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- Created a "dog task" (woof woof, unsecure) and a "duck task" (quack quack, secure).
- Viewed mail for both in `bin/mail` and web UI, saw appropriate security/encryption behavior.
- Viewed "Must Encrypt" in "Headers" tab for the duck mail, saw why the mail was…