FR:firebase client could handle retry for batch Send methods

[sahejsingh]

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Describe your environment

• Operating System version: Mac / Linux
• Firebase SDK version:
• Library version: 4.7
• Firebase Product: cloud messaging

[REQUIRED] Step 3: Describe the problem

Retry when one of the tokens gets INTERNAL error.

Here is the scenario:

• client uses SendAll to notify 10 different customers using 10 different tokens.
• say 1 of them gets internal error.

And we would like to try for that one token that got the internal error.

Proposed Solution

This library allows users to configure

• to retry on errors specified here: https://firebase.google.com/docs/reference/fcm/rest/v1/ErrorCode
• honour the Retry-After header

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