Check if action mailbox config is defined

[mullermp]

Potentially fixes #133

[mullermp]

@ssunday @bobf Sorry to bother - but do you think this will fix #133? I think it would but I think there's something smelly about this as it is currently. I tried using ActiveSupport.on_load(:action_mailbox) but to no avail. I also think maybe this engine should not be loaded all the time unless action mailbox is required. Should this also live in a railtie? Let me know what you all think.

[ssunday]

@mullermp No idea. This seems like it makes sense, but I don't know that much about railties/etc.

[ssunday]

It's a low-key problem with this gem is that it's providing all sorts of functionality even if you don't want it. Like it's pulling in aws gems that you may not need.

[mullermp]

We made the decision a few years ago to bundle all the features because it's 1) easier for us to manage (develop/release) and 2) more "all in one" and "works out of the box" like rails is itself.

I think we want to keep this pattern but I agree we can probably not bring in SDK gems as dependencies but rather check if they are loaded and enable those features as such.

[ssunday]

I'll approve but 🤷🏻