Completing NSSecureCoding Support

[k4ety]

Description

This PR addressed most of the requirements to support NSSecureCoding. In order to take advantage of this, all instances of [ decodeObjectForKey:] should be replaced with [ decodeObjectOfClass: forKey:] or its cohorts, and
- requiresSecureCoding should be implemented and respond with YES (Apple documentation).

FIRMessagingAPNSInfo.m and FIRMessagingTokenInfo.m both are currently using [ decodeObjectForKey:] with subsequent checks to make sure they got what they were expecting. While this is a start at preventing object substitution attacks, potentially bad objects are still decoded, and any security code scan still spots the insecure
[ decodeObjectForKey:] statements.

Firebase SDK Version

10.21

Xcode Version

15.1

Installation Method

CocoaPods

Firebase Product(s)

Messaging

Targeted Platforms

iOS

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