Add Google Tag Manager support for amp-subscriptions-google component

## Describe the new feature or change to an existing feature you'd like to see

We are running subscription based amp-site. Subscriptions are fully managed by Subscribe with Google (amp-subscrpitions-google compoment). Tracking runs fully on Google Tag Manager configuration.
However, we cannot track a full conversion (=succesful subscription) with Google Tag Manager.

Can you please provide a solution to collect the events defined here https://amp.dev/documentation/components/amp-subscriptions/?format=websites via Google Tag Manager?

## Describe alternatives you've considered

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## Additional context

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Start with the amp-subscriptions-google component and review the amp-subscriptions documentation linked in the issue, focusing on the events that need to be collected. Done means defining a solution that lets Google Tag Manager observe successful subscription conversions and the other documented events.

Written by the indexing model from the issue text.