Add support for validities in templates

[maraino]

This commit upgrades go.step.sm/crypto with a version that adds support for setting validity bounds in templates. It also modifies the X.509 certificate modifiers to support them. SSH certificate modifiers should work without changes.

The priority for the validity bounds are:

1. Coming from flags.
2. Coming from the template.
3. Defaults.

Requires:

• [ ] https://github.com/smallstep/crypto/pull/534