Add the attribute ExcludeFromCodeCoverage for migrations

# Context

When pushing changes to git for an application I'm working on, some unit tests are executed and pushed to Sonarqube where I can get a unit test code coverage percentage.

However, if I do nothing, the EFCore migration are analyzed and lower the code coverage of the whole application where these actually don't need to be considered as part of the code.

# Solution

Currently, I'm modifying each migration file by adding the `ExcludeFromCodeCoverage` attribute on the top of the ".Designer" class but it would be nice that the file generated with the `Add-Migration` command includes this attribute.