Comments: allow gating only to paid members or free subscribers

[simison]

To keep commenting more spam free, and within blog's reader community, a publisher might want to limit the discussions only to paid members or Newsletter subscribers.

Example from a competitor:

Currently, commenting on paid posts is limited to paid members, and open posts depending on WordPress settings (comments open, closed, for signed up users only).

Internal references

Team 51 discussion:

• p9MPsk-3Zr-p2/#comment-27309
• 26-gh-a8cteam51/public

Slack convos:

• p1686814175340449-slack-C01B6KEJ5GE
• p1730912998442399-slack-C052XEUUBL4

This might also be tangentially related (or need updating if we change how it’s working now):

• Subscriptions Management: Add comments filtering wp-calypso#75930