Monetize: Merge Subscriber login button and paid-content Login button

### Impacted plugin

Jetpack

### What

Both button have the same functionalities and have duplicated features

### How

Discussion here: https://github.com/Automattic/jetpack/pull/35441#issuecomment-1952390591

- This will allow to proxy the token on Jetpack/self-hosted (see https://github.com/Automattic/gold/issues/359 )
- We should be able to proxy the subscriber login button in the paid-content login button directly (in order to be backward compatible with existing `premium-content/login-button` blocks

Start with the discussion in Automattic/jetpack#35441 and the linked gold issue #359, then compare the Subscriber login button with the paid-content Login button. Done means their duplicated functionality is merged while existing premium-content/login-button blocks remain backward compatible and token proxying works as described.

Written by the indexing model from the issue text.