Expose random_data

[Ekleog]

The function is already implemented at https://github.com/jerusdp/mockd/blob/main/src/misc.rs#L9

It'd just be useful to expose it so users don't have to depend on simplerand in addition to mockd: I think it's a pretty often-recurring thing that's required to generate databases, to take a random id from a list of available ids.

WDYT?