[Feature request] Allow setting multiple API keys

[vassyz]

Some of our projects use multiple Notify services, requiring different API keys. The mail-notify gem doesn't seem to be supporting changing the API key as it reads it directly from ActionMailer. I solved this by updating the ActionMailer settings just before calling the view_mail method. This is not a very elegant solution and could potentially cause issues with parallel executions. Example here: https://github.com/DFE-Digital/refer-serious-misconduct/blob/main/app/mailers/application_mailer.rb Please let me know if there is a better way of making this work or if there are any plans to update the gem for this use case.