[BP to 7.x] support projector_stats in projector

As of couchbase server 7.6, projector stats are logged to projector.log file in a single json blob. In Morpheus, we are improving this by
1. Logging stats to a different file (projector_stats)
2. Exploit stats deduplication over time.

The new file proposed here will be managed by projector service. But for that projector will need help from ns_server for the following.
1. Provide the path to ns_server log directory as command line argument
2. Improve cbcollect to get projector_stats

Similar to this has been done in the past for indexer_stats as a part of MB-39281. cc: Amit Kulkarni