Support non-JSON object decodable values in getMetadataValue

[codykerns]

Checklist

• If applicable, unit tests
• If applicable, create follow-up issues for purchases-android and hybrids

Motivation

When fetching an offering metadata value with getMetadataValue(for:) (without a default value), a primitive Decodable (like String) causes the JSON decoder to crash, as it assumes the value is a JSON object, but it isn't a valid top-level JSON type.

Description

• Use JSONSerializer's isValidJSONObject method before attempting to serialize values
• If it's not a valid JSON object, attempt to return the metadata value as the provided decodable type

[aboedo]

looks great so far! We should add a test to ensure we don't break it again in the future and then ready to merge

[aboedo]

Thanks for taking care of this!!

[nyeu]

Hey @aboedo is there a reason why getMetadataValue<T>(for key: String, default: T) -> T is not set as Decodable? I was about to open a PR with this change but maybe there's a reason I'm missing here.

Here's what I think it should be #4561

I don't know why these 2 methods should be different while one is only providing a default value. Otherwise I feel the name should explicit the difference. Wdyt?