make length verification configurable (enable/disable)

[op-euga]

since upgrading to 7.x I always get the error "Provided key is too short" when decoding. I am using HS256 but the key is 18 chars long.
as I cannot change the key (external service) and still want to use an actual version of this lib, it would be nice to be able to disable the length check.

[bshaffer]

The latest major version of this library, version 7 was released specifically to address CVE-2025-45769, a vulnerability from improper key lengths (see #600), so the recommended solution is to downgrade to v6 or to use a longer HS256 key