Track and allow graphing users/groups by "date first seen"

[macobo]

Is your feature request related to a problem?

I want to track growth of my userbase.

Describe the solution you'd like

Automatically track date the user/group was first seen (#3031), allow graphing sums based on a user/group property.

Describe alternatives you've considered

• Creating a cumulative graph over all of time for "unique groups"
• Emitting a "signed up"/"first seen" event from our code

Additional context

Related: #3031

cc @EDsCODE for feature ownership

Slack thread/user request: https://posthogusers.slack.com/archives/C01GLBKHKQT/p1663928256096849

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