feat(dispatcher): add new onKnexCreated callback

[Qbit982]

Summary

Add an optional onKnexCreated callback that is called synchronously for every Knex instance created by PGDispatcher, before database health checks start.

Motivation

Consumers may need to attach metrics, tracing, event listeners, or plugins directly to every Knex instance.

Currently, Knex instances are only accessible through the primary and replica getters. These getters require a healthy database connection and can throw No connections available during application startup. They may also return different clients after a failover.

The new callback provides access to every Knex instance at creation time without depending on database availability or primary/replica selection.

Behavior

• The callback is optional and backward-compatible.
• It is called once for every configured connection.
• It runs immediately after the Knex instance is created.
• It runs before the initial database health check.
• If the callback throws, PGDispatcher construction fails.

Example

const {db} = initDB({
    connectionString,
    onKnexCreated(knex) {
        knex.on('query', handleQuery);
        knex.on('query-response', handleQueryResponse);
        knex.on('query-error', handleQueryError);
    },
});

[jhoncool]

Let's add text to README.md
No need to add a comment here

[jhoncool]

@Qbit982 typo feat(displather)

[Qbit982]

@jhoncool FIXED, see again please