Subscriptions: Prevent Password Protected Posts From Being Sent to Subscribers

#### Is your feature request related to a problem? Please describe.
If I publish a password-protected post, my intent is for it to be seen by only a subsection of my subscribers, not my entire audience. However, a password-protected post is sent via email to all subscribers with no way to turn this off.

#### Describe the solution you'd like
A way to publish a password-protected post without delivering emails notifying my subscribers.

#### Describe alternatives you've considered
Publishing a post privately will not trigger the notification email, but that requires my intended audience to register with a WordPress.com account and become an approved user of my site. That is too much friction. A password-protected post is an easier path to viewing the protected content.

#### Additional context
Requested in #22678445-hc

No files or tests are named. Start by locating the subscription email delivery path and the handling of password-protected posts, then trace how notifications are selected. Done means a password-protected post can be published without an email being sent to subscribers while other subscription notifications continue to work.

Written by the indexing model from the issue text.