ApiGatewayV2CustomAuthorizerV2Request identity_source as optional

[HVKukkonen]

Issue #, if available: None

Description of changes:
Fixes typing issue with ApiGatewayV2CustomAuthorizerV2Request not allowing for None as the identity_source. I suspect this being an issue as per the docs

When caching is turned off, it is not necessary to specify an identity source. API Gateway directly passes the request to the authorizer Lambda function.

Also, I have run into issues with this in the wild as not having the identity source in the API Gateway Terraform configuration results in DeserializeError { inner: Error { path: Path { segments: [Map { key: \"identitySource\" }] }, original: Error(\"invalid type: null, expected a sequence\" in my Authorizer code. And after specifying an arbitrary field from the API Gateway event as the identity source it worked fine.

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[bnusunny]

Could you add a test to cover this case?

[HVKukkonen]

Added. Of course, nothing guarantees that the fixture used exactly matches the event from the API Gateway but that's the same issue with all of the fixtures, right? 🥲

[bnusunny]

Could you fix the linting errors?