Unify implementations for `GetLaunchSettings` between `Aspire.Hosting`, and `Aspire.Hosting.Testing`

[radical]

The core logic is the same but duplicated in these two methods:

https://github.com/dotnet/aspire/blob/357a0b2e35b9188ba7c4437be3287b9b86388176/src/Aspire.Hosting.Testing/DistributedApplicationFactory.cs#L194-L218
https://github.com/dotnet/aspire/blob/357a0b2e35b9188ba7c4437be3287b9b86388176/src/Aspire.Hosting/LaunchProfileExtensions.cs#L55-L95

context: #5218 (comment)

[davidfowl]

Any reason we're not doing source sharing? We may also want to expose this to other integrations. Azure functions needs this logic as well.