Make getMetadataValue with default value decodable

[nyeu]

Motivation

I think we'll always want to have a Decodable value when trying to get a value from metadata. This also makes it go with the other function getMetadataValue<T: Decodable>(for key: String) -> T? but with the difference to provide a fallback value. We are not duplicating code and avoids having the crash issue we had before.

Checklist

• If applicable, unit tests

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