RemoteConfig ignores missing defaults plists

### Description

If you point RemoteConfig at a plist file and that file isn't found at runtime, RemoteConfig logs an error to console, but doesn't throw.

Then all attempts to read with configValue(forKey) just return default primitive values, e.g. 0 for numbers. That's very dangerous if you're using this for example to set a minimum time before something is deleted, or to configure the frequency with some operation can happen.

A refactor at any time could cause the plist file to fail to be found, which could then easily go unnoticed if the things that depend on remote config aren't immediate and obvious.

A method that throws or even panics would be preferable.

The whole API seems a bit dangerous. Setting the defaults in bulk when you initialize RemoteConfig, rather than at the call site forces you define the key multiple times, which introduces a not insignificant risk that you either forget to set the default or mistype the key name somewhere. I'd prefer a `configValue(forKey: <>, withDefault: <>)` method over `setDefaults` so that I'm guaranteed the ability to set a reasonable default value. Then the worst case scenario is that I use my sane hard coded default instead of my better remote default. Today the worst case seems to be I end up using a risky primitive default value.

### Reproducing the issue

Point remote config at non-existent file via https://firebase.google.com/docs/remote-config/get-started?platform=ios#rest_2

### Firebase SDK Version

10.29.0

### Xcode Version

15.0.1

### Installation Method

Swift Package Manager

### Firebase Product(s)

Remote Config

### Targeted Platforms

iOS

### Relevant Log Output

_No response_

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