fix(apigatewayv2-integrations): integrations of imported lambda functions do not get configured (#27861)

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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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: sumupitchayan