Fix type for claims_to_add_or_override in CognitoEventUserPoolsPreTokenGenV2

[AbhigyaKrishna]

📬 Issue #, if available: #1099

✍️ Description of changes:
claims_to_add_or_override now has the correct type for overriding complex structure in claims. Pre token generation Lambda trigger V2_0 allows to pass complex datatypes to ID and access token claim values.

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• I confirm that I've ran cargo +nightly fmt.
• I confirm that I've ran cargo clippy --fix.
• I confirm that I've made a best effort attempt to update all relevant documentation.
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[jlizen]

This will be flagged as semver breaking, but the argument is that the previous case was a bug and would lead to incorrect deserialization failures? (Or would it? I think no, it would just cast everything to String)?

According to the bug argument will need to bypass cargo semver check for it I guess?

But, I'm wondering if we should instead be preserving old String and offering a new API for json? This seems parallel to: #977

[jlizen]

I reread the upstream docs, and they are very explicit that this field is a complex object:
https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-lambda-pre-token-generation.html#cognito-pretoken-complex-objects-note

And, v1 explicitly does not have the complex type, so this is likely something that was present since the start of the protocol version.

So - sorry to anybody that this might break, but this is a bug that needs to be fixed, albeit very belatedly. It is blocking conventional use of the field for future users.

Thanks for the contribution, @AbhigyaKrishna !

[jlizen]

Semver check failures are expected as this is a bugfix of a public type.