Add WorkflowType enum with feedback-survey support

[facumenzella]

Port of RevenueCat/purchases-ios#6815

Summary

• Introduces WorkflowType enum (PAYWALL, WEB_FUNNEL, FEEDBACK_SURVEY, UNKNOWN) to WorkflowModels.kt
• Adds optional workflowType: WorkflowType? field to PublishedWorkflow so the Android SDK can identify feedback-survey workflows from the backend response
• Unknown values decode to UNKNOWN via EnumDeserializerWithDefault with an explicit key map (required because "web-funnel" and "feedback-survey" use hyphens, not underscores)
• Missing field decodes as null for backwards compatibility

Deviations from iOS

• No warning log on unknown type — follows the existing Android pattern (WorkflowTriggerType also silently falls back to UNKNOWN)

Test plan

• WorkflowTypeDeserializationTests covers all 3 known types, unknown type → UNKNOWN, and future unknown string → UNKNOWN
• PublishedWorkflowTest covers JSON decoding of all 3 known types, unknown string → UNKNOWN, and missing field → null
• ./gradlew :purchases:testDefaultsBc8DebugUnitTest --tests "com.revenuecat.purchases.common.workflows.*" passes

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