pass the secret to fastify cookie if directly registered

[gurgunday]

Fixes https://github.com/fastify/fastify-cookie/issues/227

I feel like this is a better solution than 238 since it doesn't degrade security by silently not signing a cookie

Also, if fastify-cookie is manually registered before secure-session (in which case the user MUST pass a secret if they expect to sign their cookies afterward), it still throws indicating an incorrect usage

Checklist

• [x] run npm run test and npm run benchmark
• [x] tests and/or benchmarks are included
• [ ] documentation is changed or added
• [x] commit message and code follows the Developer's Certification of Origin and the Code of conduct