Handle signed cookies

[JohanManders]

Before this commit, when using signed: true inside the cookie settings for fastify-secure-session, the cookie was passed without unsigning the cookie, so it would always be invalid. After this commit, signed sessions should work fine.

I added a test file to show it works and that tampering the cookie will invalidate a signed cookie. The test will fail in the old version.

Checklist

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

[mcollina]

lgtm

[gurgunday]

lgtm