Add GpfItineraryLayer tool

[LionelZoubritzky-IGN]

Description

Add an itinerary layer tool, which returns the GeoJSON and the associated distance and travel time.

Related issues

Close #145

Motivation

Test: "Show me the route from Dunkirk to Menton"

Implementation

Reuse the existing navigation/itinerary machinery.
Input constraints:

• when asking for pedestrian mode, departure and arrival must not be more than 200 km apart. This reflects an upstream limit (maybe more around 250 km upstream? I don't see it in the documentation but it happens in practise);
• no crow-flies distance limit for car mode, as the Dunkirk-Menton example above illustrates (it works in a few seconds);
• departure and arrival must be within the bbox accepted by bdtopo-valhallala.

Testing

npm run build && npm run verify -> 642+4+35 tests all green.

Checklist

• The PR is focused and of a reasonable size.
• The commit history is clean.
• Relevant documentation has been updated.
• Relevant tests have been added or updated.