Break up push adapter APIs

[mtrezza]

New Feature / Enhancement Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.

Current Limitation

With web, expo, APNS and Firebase push support, the push adapter forces additional dependencies on deployments that may not need it. In fact, some of these APIs are mutually exclusive.

Feature / Enhancement Description

Break up the push adapter into individual adapters per API, so that developers add only push support for APIs that they actually need.

Similar to database adapters. This requires a change on the Parse Server side as well.

[parse-github-assistant]

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