fix: Makes two properties optional on MigrateUserResponse.

[mark-keaton]

Issue #, if available:
No issue available

Description of changes:
Makes two properties Optional on CognitoEventUserPoolsMigrateUserResponse:

• desired_delivery_mediums
• force_alias_creation

How I diagnosed:
My Cognito Lambda was panicking when trying to deserialize the Lambda Event:

ERROR Lambda runtime invoke{requestId="..." xrayTraceId="Root=...;Parent=...;Sampled=0"}: Error("invalid type: null, expected a boolean", line: 1, column: 420)

I used Python to print the Event, and I found that all of the fields on the CognitoEventUserPoolsMigrateUserResponse were null:

{
  "version": "1",
  "triggerSource": "UserMigration_Authentication",
  "region": "us-east-1",
  "userPoolId": "us-east-1_123abc",
  "userName": "test@test.test",
  "callerContext": {
    "awsSdkVersion": "aws-sdk-unknown-unknown",
    "clientId": "123abc"
  },
  "request": {
    "password": "Testtest",
    "validationData": {
      "custom": "property"
    },
    "userAttributes": null
  },
  "response": {
    "userAttributes": null,
    "forceAliasCreation": null,
    "enableSMSMFA": null,
    "finalUserStatus": null,
    "messageAction": null,
    "desiredDeliveryMediums": null
  }
}

So I believe the deserialization was failing on forceAliasCreation. Then when I ran this event locally, it also failed to deserialize desiredDeliveryMediums.

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I confirm that I've made a best effort attempt to update all relevant documentation.

[christopherwxyz]

The modified code includes the following changes:

1. desired_delivery_mediums field is modified to Option<Vec<String>>. The Option type is added, allowing the field to be Some if present or None if absent.
2. force_alias_creation field is modified to Option<bool>. Similar to desired_delivery_mediums, the Option type is added, making it optional.

By adding the Option type to both fields, the code allows these fields to be omitted or set to None when constructing an instance of the CognitoEventUserPoolsMigrateUserResponse struct. This change provides flexibility in handling scenarios where these fields might not always be present or need to be explicitly set to None.