AB#289456 Update session length handling in Overlay Messaging

[stuart-reid-commits]

1. Add a sessionLengthMinutes overload to OptimoveConfigBuilder.enableOverlayMessaging(...) so customers can configure overlay messaging session windows down to 15 minutes, without changing the existing hours-based API.

• OptimoveConfig.swift: added enableOverlayMessaging(sessionLengthMinutes: Int) (min 15) alongside the existing enableOverlayMessaging(sessionLengthHours: Int = 1) (unchanged, min 1). Both now funnel into an internal overlayMessagingSessionLengthMinutes: Int field.
• OverlayMessagingSessionManager.swift: init now takes sessionLengthMinutes: Int, converting directly to seconds (* 60.0).
• OptimoveOverlayMessaging.swift: storage/param renamed to sessionLengthMinutes: Int to match.
• CHANGELOG.md: documented the new overload.

2. reset overlay session when user associated

Counterpart for optimove-tech/Optimove-SDK-Android#102