Ensure maxTimeToLive is bigint and not number

[nmattia]

It looks like some clients (in particular taggr) use a number maxTimeToLive in the authentication request, whereas Internet Identity expects a bigint.

For now we'll be lenient and accept bigint as well as number but we should ensure all our client libraries enforce bigint.

Note: as we make the interface more strict again we should add a test suite.