Types: Actually add "pg" property to FastifyInstance (semi-breaking)

[voxpelli]

Follow up to #68.

This actually adds PostgresDb as the type of fastify.pg by default – and it adds Record<string, PostgresDb> as the default as well.

This means that anyone trying to access fastify.pg will get the correct types, with any additional property outside of the default ones being exposed as possible multi DB:s.

What do you think @mcollina @fox1t?

This definition will conflict with anyone who has made a local definition of the fastify.pg property, as was previously recommended, so from a types point of view this should probably require a 4.0.0 release?

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, I'd be happy to land this in a semver-minor.

[mcollina]

I've seen types break in patch, minor and everything, as well as TS shipping breaking changes in minors. I'm not really concerned, the TS community is pretty loose on semver.

[fox1t]

I really like this implementation!
LGTM!

[fox1t]

I agree to ship this in a minor, as usual.