FR: Allow users to override the client config

[agnivade]

[REQUIRED] Step 2: Describe your environment

• Operating System version: linux
• Firebase SDK version: latest
• Library version: v4.12.0
• Firebase Product: messaging

[REQUIRED] Step 3: Describe the problem

The entire HTTP client is an internal package. There is no way to override the retry logic nor specify the max number of retries. This makes it very hard to have more control over the client.

It seems like everything is abstracted away with very little control given to users of this library. I am not sure why it's designed like this, but this makes it hard to use the library to its full extent.

Somewhat related: #576