Require a `radius` parameter other than `unlimited` when using `bearings`

[TheMarex]

Currently every plugin will search the whole StaticRTree until a matching bearing segment is found - no matter how far away it is. For a lot of users this is an unexpected result and can have serious performance implications (even on accident).

Hence to make the user aware that it actually only makes sense to search in a specific search radius, say 100m when using the bearings filter, we should enforce this on the API level.

This means in case of no segment with the given bearing value in the search radius we will return NoSegment which is a better indicator then just returning a coordinate potentially across the Atlantic.

Two options for implementing this:

1. bearings without setting radius will error (invalid request)
2. bearings implies that radius=SOME_DEFAULT_VALUE

I would prefer option 1 because it makes it more explicit that we changed the behavior.

[chaupow]

I guess in most cases when the segment with a matching bearing is too far away, the range of the bearing is probably too strict/narrow/small. Currently, we also do not recommend any value that should be used as a bearing range in osrm. We should at least recommend some range that makes sense in the osrm documentation (is 90 a good range? and a short sentence explaining range would be nice too...)

per chat with @TheMarex:
It might also make sense to decide on a default range X for bearings and use that default value. Three options for implementing this:

1. change the bearings option to bearings={value} which works like bearings={value},X
2. accept bearings in both forms, bearings={value},{range} and just bearings={value} which works like bearings={value},X
3. leave it as it is

I like option 1 as it makes things easier but I don't know if there are many use cases where people would want to set specific ranges.

Otherwise having a radius definitely sounds good - especially considering performance.

@1ec5, @freenerd, @danpat any opinions on this?

[1ec5]

I don't have a strong opinion on the syntax changes proposed here; libraries such as MapboxDirections.swift rationalize away any syntactic problems anyways. Rather, I think most of the confusion stems from bearing falling into an uncanny valley that's just different enough from the expected behavior, but coupling with a radius seems to address that.

[whytro]

I'd like to take a crack at this if this issue is still open and available.
Unless I missed something, it looks like it should only require changes to the parameters' validation function/help message generation, as well as the corresponding adjustments for each service.
This would be for the first option of "bearings without setting radius will error", with the goal of causing such requests to return an InvalidOptions code.