fix: deserialization of custom scalars

[jellybeansoup]

Issue #, if available:
None.

Description of changes:
We use a number of custom scalars with AWS AppSync in our codebase, which are successfully parsed from the AppSync service, and get stored within the SQLite cache, but fail to be deserialised and thus cause cache reads involving these types to completely fail. This has primarily meant that we are unable to cache these calls at all, relying on live data for calls relying on our custom scalar types.

This is exactly the same as an issue raised on the Apollo library in 2018, and was resolved by returning the fieldJSONValue from the SQLiteSerialization.deserialize(fieldJSONValue:), which correctly returns the custom scalar value as-is (in this PR).

I've replicated the changes here, including the removal of the then defunct AWSAppSyncQueriesCacheError.invalidRecordValue case.

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[atierian]

Thanks for opening this PR (and the other one). We'll review and come back to you on this.

[lawmicha]

Thanks for opening this PR @jellybeansoup. Let's verify integration tests are passing as expected (SDK owners can assist in running this if it's difficult for developer contributor to perform this)

[lawmicha]

From the Apollo issue linked in the description, has the PR with the same change to the SQLiteSerialization file here https://github.com/apollographql/apollo-ios/pull/1144/files#diff-e9bbcdac7903360aed17a5df82e644121e800adbc8b56bc03ffcf1dccc7dfe0a . With this, this change looks accurate to do since I think the file was lifted from the apollo codebase.

[jellybeansoup]

Let's verify integration tests are passing as expected (SDK owners can assist in running this if it's difficult for developer contributor to perform this)

I'd certainly appreciate it if someone could assist; I had trouble getting the appropriate AWS environment set up (the lambda instructions in particular), but I did verify that the other tests run and pass as expected.

[lawmicha]

Let's verify integration tests are passing as expected (SDK owners can assist in running this if it's difficult for developer contributor to perform this)

I'd certainly appreciate it if someone could assist; I had trouble getting the appropriate AWS environment set up (the lambda instructions in particular), but I did verify that the other tests run and pass as expected.

Ran the integration tests and they're all passing

[jellybeansoup]

Thanks for running the integration tests on my behalf, @lawmicha! I've cherry-picked the commit to return the removed error case as suggested, so it should be good for another look 😊