R1:af71c51f0a0b

Give "MetaMTAMail" a "message type" and support SMS

Summary:
Depends on D20011. Ref T920. This change lets a "MetaMTAMail" storage object represent various different types of messages, and makes "all" the `bin/mail` stuff "totally work" with messages of non-email types.

In practice, a lot of the related tooling needs some polish/refinement, but the basics work.

Test Plan: Used `echo beep boop | bin/mail send-test --to epriestley --type sms` to send myself SMS.

Reviewers: amckinley

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