Creative Mail: configure some options when activating plugin

[jeherve]

Built on top of #16818, do not merge for now. We can change the base branch once the other PR has been merged.

Changes proposed in this Pull Request:

• When installing the plugin via Jetpack, automatically configure some plugin options.

Note

I was originally hoping to use the activated_plugin action provided by Core, but it then wouldn't be possible to pass a source that we can use in the option. I consequently ended up creating a new hook (I added the docblock in #16821), that we can also use when activating the plugin via the plugins endpoints introduced in #16713.

Jetpack product discussion

• Internal reference: pbtFPC-H5-p2

Does this pull request change what data or activity we track or use?

• No

Testing instructions:

• Similar setup as in Compat: add new Creative Mail compat file #16818
• Upon plugin activation, run wp option get CE4WP_REFERRED_BY
• You should see the following data being returned:

array (
  'plugin' => 'jetpack',
  'version' => '8.9-alpha',
  'time' => 1597304061,
  'source' => 'jitm',
)

Proposed changelog entry for your changes:

• N/A