fix(apigatewayv2-integrations): integrations of imported lambda functions do not get configured

[sumupitchayan]

Closes #18965

HTTPLambdaIntegration using imported lambda functions are currently not being configured. This is due to the fact that the canCreatePermissions property is set to false in imported lambdas, so the lambda permissions are never created.

This PR fixes this issue by:

• Setting the sameEnvironment property to true for lambdas imported using fromFunctionName()
• Adding clarification in documentation about imported lambdas needing to belong to the same stack account and region as that of the lambda being imported from be able to add permissions
• Adding a new warning to be thrown when the addPermissions() function is called on a lambda with canCreatePermissions set to false.

The new integ test file tests that imported lambdas using fromFunctionName() and fromFunctionAttributes() (with sameEnvironment set to true) work as expected, including on sub-route integrations.

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• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: c6d87e5
• Result: SUCCEEDED
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Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).