ENH: Add observed keyword to value_counts

[jseabold]

Is your feature request related to a problem?

If you do value_counts on a categorical there's no way to suppress unobserved categories.

Describe the solution you'd like

Add an observed keyword to value_counts.

API breaking implications

None, unless we want to revisit the default in light of #35967.

[usersblock]

Hello, is there a code sample you want to add to testing?

[jbrockmendel]

closing as duplicate of #46486 (even though this came first, there is more discussion there)