whitelist functionality

[kumarankitapp]

This Pull request is functionality addition to Whitelisting feature for Describe functions.
It adds a dedicated dynamodb table record for every issue that needs to be whitelisted across all describe functions. To achieve this, we create a new dynamodb table (whitelistddb) that stores account and issue ids pointing to the actual issue in different rule tables.

1. Each entry in whitelist table is made unique by concatenating issue id with the rule name it corresponds to. This eliminates the duplicates in case of similar keys in various rules.
2. Each entry in whitelist ddb is considered whitelisted by describe functions which first check the existence of open issues in this table and ignores them if they exist here in whitelist ddb.
3. As soon as the describe function encounters and open entry in this DDB table, it updates the status value in its rule ddb record to whitelisted.

This functionality allows whitelisting based on user feedback (through Jira ticket, Slack, etc.).
It also allows dynamic addition and removal of issues from the whitelist.
It supports the existing whitelist.json entries.