Comments: allow gating only to paid members or free subscribers

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):
- https://github.com/Automattic/wp-calypso/pull/75930

No files, tests, or code entry points are named. Start by tracing the existing paid-post and WordPress comment-settings behavior, then clarify the desired subscriber and paid-member states and define tests for each commenting access option.

Written by the indexing model from the issue text.