allow connection to self hosted instance

[draeron]

Just remove the hardcoded api url to a default/provided one

https://github.com/openstatusHQ/terraform-provider-openstatus/blob/main/internal/provider/provider.go#L63

[thibaultleouay]

@draeron Would love if you provide a pr :)

Have you been able to self host openstatus ?