Change field type of url field

[tester22]

The field for the url (blackbox exporter probe) is of type URLField. This prevents the blackboxexporter to be used for tcp, ssh and smtp probes.

https://github.com/line/promgen/blob/209180f3b934f916e7b9a480dc4ab7fbc110cc33/promgen/models.py#L381

Changing it to CharField would introduce less validations but the fuctionality would be more flexible.

[kfdm]

I think internally we're only using blackbox exporter as an http check, hence using URLField, but you're correct that blackbox_exporter supports other kinds of checks as well. It's something that might be worth fixing, but we would also probably want to clean up the labels suggesting that it's an http only check.