Add `generateInterfaces` option to generate API interfaces for testing

[halotukozak]

Description

Add a generateInterfaces (Boolean, default false) option. When enabled, the generator should produce an interface per API tag containing all endpoint method signatures. The generated client class would implement this interface. This enables easy mocking in tests without third-party mocking libraries.

Acceptance criteria

• JustworksSpecConfiguration DSL supports generateInterfaces property
• When enabled, an interface is generated per API tag with all suspend function signatures
• Generated client class implements the corresponding interface
• Default is false — no breaking change
• Tests cover generation with and without interfaces
• README documents the option