PERF-1885 unique index benchmark

[wh5a]

Jira Ticket: PERF-1885

Whats Changed

A new workload that tests performance of unique indexes, in particular concurrent insertion of duplicate keys. It also tests concurrent update and deletion of documents using the indexes, starting from 8 threads up to 128 threads.

Patch Testing Results

https://spruce.mongodb.com/version/662d950c48f968000712eaf6/tasks?sorts=STATUS%3AASC%3BBASE_STATUS%3ADESC