fix: simplify power_pin configuration

[Rapsssito]

Working on oarcher/esphome-docs#2, I realized that the configuration for the power_pin from #5 was redundant and could be simplified.

[oarcher]

Many thanks for your contribution, @Rapsssito — much appreciated!

However, could you please split this PR?

For this PR, I’d like to keep only the changes under components/modem.
The modem component is already quite large, and it will be much harder for core developers to review and merge it if it also modifies multiple unrelated components.

More specifically:

• The change around checking whether esp_event_loop_create_default() was already called (handling ESP_ERR_INVALID_STATE) affects several core components. This should probably be proposed as a separate PR against the main ESPHome dev branch, and discussed directly with the core developers. If accepted, we can then rely on it from the modem component.
• Same idea for the change in components/network/util.cpp, but it can probably be skipped, as it’s just a semantic rewrite.

[Rapsssito]

@oarcher, yeah I also worried about that. I have removed those parts from this PR and created a new one in the main repo (esphome#13896).

[Rapsssito]

@oarcher, I have removed everything related to multiple interfaces to just simplify the power pin config and match the new docs (oarcher/esphome-docs#2).

[oarcher]

Thanks for your help!