Move Provider._enforce_field_name_rules out of Provider

[BrianJKoopman]

The Registry currently depends on so3g/spt3g because it uses Providers to check field name rules:

ocs/agents/registry/registry.py

Line 169 in 58f7e3f

field = Provider._enforce_field_name_rules(field)

This function should be moved to some general location so that this dependency on the Registry can be removed.

Note: When this is done, tests for the Registry can be unmarked as dependent on spt3g/so3g.