Add ability to change load strategy in whole application, like a global option

[ceopaludetto]

Describe the solution you'd like
Will be much helpful, change the default load strategy behavior globally.

Talking about performance, the benchmarks are case specific, this occurs because some drivers are more performant in join statements and anothers are more performants in muiltiple statements, database structure and indexes can affect directly in this result too.

Some discussions I found searching this topic:
https://dba.stackexchange.com/questions/42998/are-individual-queries-faster-than-joins

The option chain can be:

Global option -> Model declaration option -> Query option

Mongo would ignore this option.

Describe alternatives you've considered
A global option like today naming strategies in init static method:

const orm = await MikroORM.init({
  loadStrategy: LoadStrategy.JOINED,
});