Refactor code to improve static type checking (DomainEntry dataclass)

[pdcastro]

This PR does not provide any user-visible features, but it provides two development benefits:

1. It replaces the computation of the following dictionary keys (string concatenation), which was found in copy-and-paste form across 6 files:

we_connect = hass.data[DOMAIN][config_entry.entry_id]
coordinator = hass.data[DOMAIN][config_entry.entry_id + "_coordinator"]
vehicles = hass.data[DOMAIN][config_entry.entry_id + "_vehicles”]

... with a DomainEntry dataclass that allows the following usage:

domain_entry: DomainEntry = hass.data[DOMAIN][config_entry.entry_id]
we_connect = domain_entry.we_connect
coordinator = domain_entry.coordinator
vehicles = domain_entry.vehicles

In the original form, the IDE (VSCode + Pylance) and linters / type checkers like pylint and pyright could not tell the types of variables we_connect, coordinator and vehicles, which were deemed ’unknown’ and affected a chain of type checking wherever those variables were used. In the second form proposed in this PR, type checking is available.

2. It adds more type hints in several selected functions and several selected files, which again allows the IDE / pylint / pyright to aid with type checking.

Tested with Home Assistant version 2024.1.6.