add API "GET /api/v1/push/subscription"

[tateisu]

related Push Subscription API #7445 .

The endpoint callback may cancel subscription by returning 404 or 410, but the mobile app client can't know about it.
( There are several reasons for the occurrence of cancellation, but I will not discuss it here. )

Currently my app periodically re-subscribe or unsubscribe because we can't know subscription state from outside of instance.

It would be nice to we have "GET /api/v1/push/subscription" to check current subscription state from instance.