customers: health check should decrypt account number

[adamdecaf]

PayGate Version: v0.9.0-dev

What were you trying to do?
Currently the Customers/Accounts health check does not attempt to decrypt the account number. We should do this (perhaps just once?) to ensure the full suite of API calls works. Decrypting account numbers is a common code flow in PayGate and ensuing the decryption keys are valid is a good health check.

https://github.com/moov-io/paygate/blob/v0.9.0-dev1/pkg/customers/health.go#L28-L38

What did you expect to see?
PayGate should make it obvious when its performance is known to be degraded.