Don't delete LegacyFCM yet

[Sinc63]

Is your feature request related to a problem? Please describe.
My project members missed the advance notifications about the deprecation of the legacy API. I've been scrambling for 2 months to correct it. In our environment we have a central server that passes pushes to Apple and Google, created so that the Apple certificates only have to be located in one server, not on all our customer servers. I have therefore taken the approach to accept incoming Legacy notifications on this central server, convert them to V1, send them to Google, and convert the responses back to Legacy. This saves me from having to convert my entire installed base of customer premise systems to a new version (some of whom are a few releases behind), while still getting current with the Google side.

Describe the solution you'd like
Don't delete the LegacyFCM code when you release 2.0. I still need those structures to manage the incoming messages and returned responses. Wait a while to remove them, maybe in a 2.1 type release, not a 2.0. This also gives you less work to do before releasing 2.0, a highly desirable result.

[hpopp]

I think perhaps a better solution here, I'll move LegacyFCM functionality to a new repo and publish a new package for it. Pigeon 2.0's adapter style was designed exactly for things like this.

Should be as simple as adding a new dep to your mix.exs. I'll keep you posted.

[hpopp]

RC3 and the legacy adapter have been published 🎉

Update your mix.exs accordingly and everything should compile fine:

def deps do
  [
    {:pigeon, "~> 2.0.0-rc.3"},
    {:pigeon_legacy_fcm, "~> 0.1.0"}
  ]
end