fix: support string or [string] when deserializing `IamPolicyStatement`

[aesterline]

Action and Resource allow both string and [string] as values. Support deserializing IamPolicyStatement with either of these values.

Issue #, if available: fixes: https://github.com/awslabs/aws-lambda-rust-runtime/issues/853

Description of changes: I thought about changing the type of the action and resource fields to be another type that was an enumeration, but it felt like a backward breaking change and I wasn't sure that was acceptable. It seems like there are other cases of deserialize_with, so I thought I would give that approach a shot.

I'm happy to try a different way (enum or other) if that is preferable.

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[calavera]

The examples failure is unrelated to these changes. I'm fixing it in https://github.com/awslabs/aws-lambda-rust-runtime/pull/855