FTS - introduce an option to delete index contents by Querying

This feature let users delete the query matched contents from an FTS index.

This could prove useful in cases where - the customer wants to retain the original documents in the KV, but for reducing the TCO on the fts index they need to clean up a few older contents or contents which became really old and no longer needs to be searched with FTS.

Another lesser attractive use case would be that - it can be used as a work around for supporting TTL to the indexed contents. Just that customer needs to schedule a cron job to hit this deleteByQuery endpoint with the desired query. Index would get trimmed accordingly. 

This approach seems simpler and effective for an actively indexed/lively index.

New endpoint could look like 
POST /api/index/{indexName}/deleteByQuery
{

}