Jetpack Compatibility: make block removal filterable

[jeherve]

Is your feature request related to a problem? Please describe.

In #1337, Jetpack's Subscriptions block was disabled to avoid confusion with Newspack Newsletters. While this works well, in some scenarios some site owners may still prefer to use Jetpack Subscriptions instead. In those scenarios, they have no alternative but to manually edit the plugin files.

Describe the solution you'd like

It would be nice to make that filterable, so folks could use a filters in a functionality plugin to restore Jetpack Subscriptions functionality on the site.

Additional context

Discussion in this thread:
https://wordpress.org/support/topic/how-to-add-newsletters-block-to-a-page/