fix: adding check for the maximum payload size

[geekbrother]

Description

This PR implements a check for the maximum payload size, limiting the size to 4Kb, and throws an appropriate message if exceeding.
The reason for limiting the body payload size is the FCM support payload up to 4Kb.
This should fix the following error:

FcmV1(UnknownHttpResponse { status: 400, body: Ok("{\n  \"error\": {\n    \"code\": 400,\n    \"message\": \"Android message is too big\",\n    \"status\": \"INVALID_ARGUMENT\",\n    \"details\": [\n      {\n        \"@type\": \"type.googleapis.com/google.rpc.BadRequest\"\n      },\n      {\n        \"@type\": \"type.googleapis.com/google.firebase.fcm.v1.FcmError\",\n        \"errorCode\": \"INVALID_ARGUMENT\"\n      }\n    ]\n  }\n}\n") })

How Has This Been Tested?

Not tested, but should be covered by the unit and integration tests.

Due Diligence

• Breaking change
• Requires a documentation update
• Requires a e2e/integration test update

[nopestack]

Why not make that inner string part of the display implementation provided by thiserror and you just embed the size of the payload in kb?

[chris13524]

This is not really an appropriate fix IMO. This is blaming the relay for the payload being too big but the relay is just forwarding messages so it doesn't address the root issue.

Why is the payload too big?