Make background validation performed by `Services` configurable

[hannobraun]

Validation uses parameters defined in ValidationConfig to determine whether close-by objects are considered coincident or not. However, this is not exposed through the Services API, which is the main way of using validation.

It should be possible to override the configuration used by Services in some way.

[hannobraun]

I think this might be a good first issue for anyone interested in getting involved. Here's some more explanation and guidance on how to get this done:

• Services is currently the entry point to the fj-core crate.
• You can see examples of its use in the example models ([1], [2], for example) or pretty much any unit test within fj-core (like this one).
• One service within Services is the validation service, which uses the default ValidationConfig by calling Validate::validate.
• So to address this issue, one would have to optionally accept a ValidationConfig in Services (maybe through a second constructor), pass that config into the validation service, and use it there via Validate::validate_with_config.

If anyone still has questions, please don't hesitate to ask!

[hannobraun]

Addressed in #2150.