Adding interface for scan by keyrange and index

[sanjanasandeep]

Discarding #14541.

We need these functions in LCM for:

Scan the table based off primary key prefix (namespace id and app name) and index field (latest == true) -> to retrieve latest version of the app. Scan the table based off primary key prefix (namespace id and app name) and sort based on index field (created time DESC) -> to retrieve the versions of the app in the GET /apps endpoint.

[gitpod-io[bot]]