sdk should add fallback public keys

[karrui]

Used to seamlessly switch to a second set of secret keys in the (miniscule?) event that the current secret keys are leaked, without affecting users.

Basically the point is:

If any secret keys are leaked, FormSG will switch to a second fallback key kept in cold? storage, which has had its corresponding public key exposed in the SDK as a fallback.

FormSG, when using the SDK, will encrypt with the fallback secret key, in which all users of the SDK should still be able to verify with the fallback public key without any work on their part using the SDK.