Types: Add "pg" property to FastifyInstance

[voxpelli]

As recommended in the guide for creating TypeScript compatible Fastify plugins, properties that the Fastify instances are decorated with should also in the types get added to those instances: https://www.fastify.io/docs/latest/TypeScript/#creating-a-typescript-fastify-plugin

This change follows up on #63 by adding that.

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

[voxpelli]

Additional prior art: https://github.com/fastify/fastify-swagger/blob/6c17b3b13278dc663dc2d55c617fd3a77d0f0919/index.d.ts#L4-L26

[mcollina]

Can you please add a unit test?

[fox1t]

Nice catch!

LGTM after adding tests.

[voxpelli]

This became more complex.

For some odd reason this module has an undocumented, yet tested and verified, capability that allows it to expose fastify.pg as either a PostgresDb directly or as a an object keyed with named database connections, each one containing a PostgresDb.

The latter is achieved by adding the module multiple times, each time adding a name.

So:

I changed course here a bit. Adding documentation for that property + tests that ensures that one always take one or the other approach.

Also clarified the intention that one should manually be adding the pg property to the FastifyInstance type.

Up next, in another PR, I would want to do a breaking change:

Have fastify.pg always be for a single db.
Have fastify.pgMulti (or similar name) always be for multiple db:s.

That way it's actually possible to expose types for both out of the box, without any users having to manually add the correct one for them.

How does that sound? Can we merge this PR as it is with the documentation and test improvements? And I'll open a new one that takes care of a possible breaking change.

[mcollina]

lgtm