InfluxDB Cloud: points limit retention policy

__Proposal:__
I suggest to provide new retention policy for buckets. Idea is to keep fixed number of last measurement points. So if new point is added and existing points count is equal to limit, oldest point from those existing points will be deleted.

__Current behavior:__
Currently we have two retention options: _delete never_ and _delete older than_.

__Use case:__
In my opinion it will be useful for Free plan for those people who haven't large amounts of uploaded data and add a couple of measurement points per hour, for example.

Let's imagine, 1 point is added per day. With Free plan we currently can keep only 30 last points (since 30 days is maximum retention period). But with new option we can store, for example, last 1000 points and keep more data for our scenario.

I'm developer of an open-source library so I don't get any money for this work and want to use Free plan. My amounts of uploaded points is pretty small I suppose, so the new retention policy will be extremely useful.

The issue does not name files, tests, or entry points. Start by locating the bucket retention-policy implementation and its tests, then review how the existing delete-never and age-based options are handled. Done means a fixed point-count policy is specified, implemented, and covered by tests.

Written by the indexing model from the issue text.