feat(lambda): add addMinInvocationsCountAlarm (#259)

Requested by a customer.

This does leave an `addMaxInvocationsCountAlarm` as a future
possibility, but I left that out since there hasn't been ask for it (and
I'm not sure if that's something that'd be actually useful). If someone
wants it, please do request it!

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under the terms of the Apache-2.0 license_

Author: Eugene Cheung