Delegated choice of DriveLetter to WDL and made internal Set immutable

Delegated choice of DriveLetter to WindowsDriveLetters
Made internal Set ("C_TO_Z") immutable, that is returned by getAvailableDriveLetters()
Changed direct reference to C_TO_Z to use getAvailableDriveLetters() instead

main/commons/src/main/java/org/cryptomator/common/vaults/WindowsDriveLetters.java

@@ -12,6 +12,7 @@ import javax.inject.Inject;
 import javax.inject.Singleton;
 import java.nio.file.FileSystems;
 import java.nio.file.Path;
+import java.util.Optional;
 import java.util.Set;
 import java.util.stream.Collectors;
 import java.util.stream.IntStream;
@@ -24,7 +25,7 @@ public final class WindowsDriveLetters {
 
 	static {
 		try (IntStream stream = IntStream.rangeClosed('C', 'Z')) {
-			C_TO_Z = stream.mapToObj(i -> String.valueOf((char) i)).collect(Collectors.toSet());
+			C_TO_Z = stream.mapToObj(i -> String.valueOf((char) i)).collect(Collectors.toUnmodifiableSet());
 		}
 	}
 
@@ -46,7 +47,18 @@ public final class WindowsDriveLetters {
 	}
 
 	public Set<String> getAvailableDriveLetters() {
-		return Sets.difference(C_TO_Z, getOccupiedDriveLetters());
+		return Sets.difference(getAllDriveLetters(), getOccupiedDriveLetters());
 	}
 
+	public Optional<String> getAvailableDriveLetter() {
+		return getAvailableDriveLetters().stream().findAny();
+	}
+
+	public Optional<Path> getAvailableDriveLetterPath() {
+		return getAvailableDriveLetter().map(this::toPath);
+	}
+
+	public Path toPath(String driveLetter) {
+		return Path.of(driveLetter + ":\\");
+	}
 }