System: extend ModuleGateway functionality

[yookoala]

Description

• Add phpdocs to existing ModuleGateway methods.
• Add method getActiveAdditional().
• Add ModuleGatewayInterface for flexibility of testing against it.
• Add ModuleGatewayInterface as a core service.

Motivation and Context

• Plan to make Locale a service loaded by container. Wanted to delegate the database query in Locale::setTextDomain to an interface.

How Has This Been Tested?

• Locally.

[yookoala]

No real objection here. I'll remove the interface then.