types: replace declaration merging with mixin types

[flakey5]

For Fastify v5, background: fastify/fastify#5061

Opened this as it is now to get early feedback & insight. Used a class rather than an interface or type since the type needs to exist at runtime if we go with the currently proposed changes to the fastify-plugin api (see fastify/fastify#5061 (comment))

TODO:

• Update/check tests.

cc @mcollina

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