feat: Implement stations screen with grouped airports

This commit introduces a new "Stations" screen that displays airports grouped by country.

Key changes:
- Added a new Gradle module: `ui:stations`.
- Created `StationsScreen.kt` to display a list of airports grouped by country using `LazyColumn`. Airports within each country are displayed as `AirportInfoItem` and countries as `CountryItem`.
- Implemented `StationsScreenViewModel.kt` to fetch and manage the state of airports grouped by country. It uses `ObserveAirportsGroupedByCountry` use case.
- Defined `ObserveAirportsGroupedByCountry.kt` use case in `domain:stations` module to provide a flow of airports grouped by country.
- Implemented `ObserveAirportsGroupedByCountryImpl.kt` in the repository layer, which fetches data using `AirportService`, stores it in `AirportsDatasource`, and maps it to the domain model.
- Added Hilt module `ObserveAirportsGroupedByCountryModule.kt` to provide the use case implementation.
- Added `stationsScreen()` and `navigateToStations()` to `FlightsNavGraph.kt` and `TopLevelDestination.kt` for navigation.
- Updated `settings.gradle.kts` and `app/build.gradle.kts` to include the new `ui:stations` and `domain:stations` modules.
- Updated `CacheObservers.kt` to make `mapToDomain` a suspend function.
- Added string resources for the stations screen title.

domain/stations/build.gradle.kts

@@ -0,0 +1,12 @@
+plugins {
+    alias(libs.plugins.flights.android.library)
+    alias(libs.plugins.flights.android.library.hilt)
+}
+
+android {
+    namespace = "dev.adriankuta.flights.domain.stations"
+}
+
+dependencies {
+    api(projects.domain.types)
+}

domain/stations/src/main/kotlin/dev/adriankuta/flights/domain/stations/ObserveAirportsGroupedByCountry.kt

@@ -0,0 +1,9 @@
+package dev.adriankuta.flights.domain.stations
+
+import dev.adriankuta.flights.domain.types.AirportInfo
+import dev.adriankuta.flights.domain.types.Country
+import kotlinx.coroutines.flow.Flow
+
+fun interface ObserveAirportsGroupedByCountry {
+    operator fun invoke(): Flow<Map<Country, List<AirportInfo>>?>
+}