fix(go): add Postgres struct constructor

[BCJuan]

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

Disclaimer New to the library and might be missing how to add plugins, but I see the following issue:

Description

Currently, the Postgres struct doesn't export the engine data member, and at the same time, it has no way of adding it through a constructor or a setter.

In detail, in the file go/plugins/postgresql/genkit.go

// Postgres holds the current plugin state.
type Postgres struct {
	mu      sync.Mutex
	initted bool
	engine  *PostgresEngine
}

And in the README.md, it says (go/plugins/postgresql/README.md)

	postgres := &Postgres{
		engine: pEngine,
	}
...

which is not possible unless you are in the same package, since the engine is not exported. Thus, this blocks the use of any instance of a Postgres struct as a plugin directly with Genkit. For example, the following won't work just because the engine is not exportable

package main

import (
	"context"
	"fmt"

	"github.com/firebase/genkit/go/genkit"
	"github.com/firebase/genkit/go/plugins/postgresql"
	"github.com/jackc/pgx/v5/pgxpool"
)

func main() {
	ctx := context.Background()
	pool, _ := pgxpool.New(ctx, "postgresql://postgres:password@localhost:5432")
	pgEngine, err := postgresql.NewPostgresEngine(ctx,
		postgresql.WithUser("postgres"),
		postgresql.WithPassword("password"),
		postgresql.WithDatabase("example_db"),
		postgresql.WithPool(pool),
	)
	if err != nil {
		fmt.Println(err)
		panic("Error creating pgengine")
	}
	postgres := &postgresql.Postgres{
		engine: pgEngine,
	}
	_ = genkit.Init(ctx, genkit.WithPlugins(postgres))
}

If one tries not to add an engine and just pass the Postgres struct instance as a plugin to GenKit, as in:

postgres := &postgresql.Postgres{}
_ = genkit.Init(ctx, genkit.WithPlugins(postgres))

will fail with panic: genkit.Init: plugin *postgresql.Postgres initialization failed: postgres.Init engine is nil error, coming from the initialization of the plugin here

Solution

Add a constructor to the Postgres struct so it can be instantiated with an engine and thus passed to a genkit plugin

func NewPostgres(engine *PostgresEngine) *Postgres {
	return &Postgres{
		engine: engine,
	}
}