[openstreetmap/openstreetmap-website] Integrate Valhalla Routing (again) (Issue #3843)

[Nils]

### Problem

Implement support for another great FOSS routing engine.

### Description

We've been hosting a public Valhalla instance for the past year, funded by FOSSGIS and maintained by our company. I just got confimation that it'll be funded in the future as well, just like OSRM. The frontend is available here: https://valhalla.openstreetmap.de/.

If you agree, we'd like to PR the implementation in this repo soon-ish (Cc @chrstnbwnkl), so it can be used on osm.org next to Graphhopper & OSRM. @kevinkreiser mentioned that Valhalla was already supported back in the days. It has been added in this commit (https://github.com/openstreetmap/openstreetmap-website/commit/e54eadea95296a68044bd3f2b092b978a808f1b7), but I guess sufficiently many changes went over both Valhalla & the code in this repo since then.

### Screenshots

_No response_

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