Implement elevation support

[xivk]

Implement elevation support similar to what was done in Itinero1.

• [x] Add an ElevationHandler to get elevation.
• [x] Store elevation on the graph.
• [x] Return elevation in the routes.

[xivk]

After implementing this we've figured out that when using elevation there are two flavors:

1. Elevation is added to the routerdb, we need it in an environment where we cannot access another source of elevation data.

2. Elevation is not added to the routerdb but used on the fly.

[xivk]

Refactor elevation handle to not be a process-wide handler but something configurable on a router/routerdb.