Support for user-defined generators

[moodmosaic]

See #7 (comment).

[mijoco-btc]

+1 here for this feature. I'm finding some tests grow in complexity when handling valid ranges for uint function call arguments whereas a way to configure the range for each argument would be a better framework level solution.

[moodmosaic]

A possible starting point is to make this configuration-based, following the configuration complexity clock and extend or reuse the ideas in #7 (comment).

[moodmosaic]

One possible approach is to let users override specific generators by type. For example:

• For integer, use customGeneratorForInteger.
• For xyz, use customGeneratorForXyz.

The mechanism could be similar to how dialers are configured.

However, to make sure rv remains safe and reliable, the user should be required to provide both:

1. A custom generator.
2. A corresponding sampler.

Requiring a sampler protects the system from faulty generators—for example, a generator that throws errors instead of producing random values.

This additional requirement helps make sure that custom implementations behave correctly before being accepted.