Implement dynamic rules

[uphlewis]

For example when registering a domain name, some TLDs require additional fields such as registrant type (individual/organisation).

This could be defined in the RegisterParams dataset rules() like so:

    return new Rules([
        // ...
        'additional_fields' => ['upmind_dynamic_rules:getRegisterAdditionalFieldRules'],
        // ...
    ]);

The argument passed to the upmind_dynamic_rules would specify where to get the validation rules from for the additional_fields object, so in this case we need to call the provider's getRegisterAdditionalFieldRules() method, which in-turn may specify which params it requires in order to return the rules we need, such as the tld.

This allows the provision system client to build a dynamic html form for domain registrations, once it knows which tld the customer wants to register a domain for, and once it has determined which provision provider to use.

Caching the dynamic rules (per provider + tld and perhaps per provision configuration) should negate any performance impact of the blocking call to getRegisterAdditionalFieldRules().