feat(index): support customOptions as object

[nicolas-goudry]

This change introduce the ability to set customOptions as an object that will be used to override the default instance options.

This aligns with the behavior of Fastify regarding ajv configuration, and fixes an issue with fastify-cli.

I've added a single test to ensure the ajv instance is passed the custom options. I updated the name of other tests in the ajv enhancement suite.

Checklist

• run npm run test && npm run benchmark --if-present
• 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