polish(auth): Add allow list for new fxa mailer sending

[dschom]

Because

• We want to feature flag the send operations with the new fxa-mailer

This pull request

• Adds a canSend method on FxaMailer
• Adds a new config setting to disable certain email templates from being sent by FxaMailer.

Issue that this pull request solves

Closes: FXA-12934

Checklist

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• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).

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[nshirley]

💯