feat: don't load webhook plug if plug isn't part of the loaded apps

[MikaAK]

Currently we cannot compile stripity_stripe anymore due to this error after upgrading to elixir 1.12 (application boundary crossing)

[info] Application plug_crypto exited: exited in: Plug.Crypto.Application.start(:normal, [])
    ** (EXIT) an exception was raised:
        ** (UndefinedFunctionError) function Plug.Crypto.Application.start/2 is undefined (module Plug.Crypto.Application is not available)
            (plug_crypto 1.2.2) Plug.Crypto.Application.start(:normal, [])
            (kernel 8.1.2) application_master.erl:293: :application_master.start_it_old/4

This PR changes the stripe webhook plug to only load if the current application has plug loaded

[snewcomer]

@MikaAK Do you have any interest in bringing in the latest master and we can see if we can get this PR's tests passing?

[MikaAK]

Done @snewcomer Tests won't run because they need approval from a maintainer

[snewcomer]

@MikaAK Can we bring in master one more time?

[MikaAK]

Sorry @snewcomer , I've done this quite a few times, I think it's best someone else does it at this point!!