Add implementation of getKeysInOffsetRange api on Attribution Collection

[jatgarg]

Description

Add implementation of getKeysInOffsetRange api on Attribution Collection. This is a requirement from Loop app to integrate the FF provided attribution. API: getKeysInOffsetRange( startOffset: number, endOffset?: number, channel?: string, ): [number, AttributionKey][] | undefined;

This api returns a list of [offset, Attribution key] pairs for the provided offset range. It helps the app to get attribution keys on a segment in a single call if they want to fetch multiple for startOffset till endOffset or from startOffset till the end of segment instead of making the app to query for key for each offset.