Allow overriding default spacing configuration

[sarialem]

Is your feature request related to a problem? Please describe.

The default separation configuration can be changed to have better arrows visibility in sequence flows, example displaying using bpmn-js:

After changing iLevelSeparation from 20 to 40:

Describe the solution you'd like

To allow overriding the default config by passing an optional config map when initializing AutoLayout

Describe alternatives you've considered

Additional context

[barmac]

The default spacing looks like a bug to me. I believe we should adopt the increased spacing as the built-in, and there's no need for custom configuration.

[sarialem]

This would work too, used your suggested approach in the PR. Probably need to label this as bug too.