Bring back option to reuse existing MikroORM instance

[askmrsinh]

Goals

1. Avoid multiple ORM instance for same DB/schema
2. Avoid possible duplicate connection to DB, exhausting connection pool
3. Increase performance by reusing inbuilt result caching on existing instance
4. This is consistent with other authjs db adapters that require an instance (e.g. fauna, sequelize, mongodb to name a few)

Non-Goals

No response

Background

PR nextauthjs/adapters#335 mentions that using existing MikroORM instance made things 'flaky' but provides no details on how so. Changes were later merged as #4008 which removed the ability to pass existing instance and instead, provide config.
I argue that using multiple MikroORM instances within the same app/request is even more problematic. In most use-cases the DB is used for more than just auth and the developer will have to create another instance of MikroORM to do so. Since MikroORM utilizes IdentityMap, and as per my understanding, those are unique to each MikroORM instance (i.e. there's no global Identity Map), this leads to bugs, degraded performance, and larger memory footprint.

cc: @boredland

Proposal

I am interested in contributing.

[boredland]

The primary reason to remove it, was to simplify it and that I wasn't even sure there are performance benefits, at least when using RequestContext on all your API routes.