Choose which symbols to explicilty export at the top level of pasqal_cloud

[HGSilveri]

In #147 , I'm marking BaseConfig, EmulatorType, Endpoints as symbols explicitly exported at the pasqal_cloud-level because these objects are being called in pulser_pasqal as pasqal_cloud.X and mypy complained about this.

However, I'm sure there are other symbols that would make sense to explicitly export so it's weird to be exporting only these three.

For clarification, there are (at least) two ways to signal a symbol as "explicitly exported":

1. Redundant imports, ie from module import X as X (this is what I did in [FEAT] Incorporate pulser-pasqal package in the pasqal-cloud repo #147)
2. Include it in the __all__ module attribute