Example App should be able to build and run without Firebase configured via google-services.json

[QuintinWillison]

Friction is way too high for those who wish to test the SDK without support for push notifications.

Relates to #316.

I observed on my local machine:

{code}
example % flutter build apk

💪 Building with sound null safety 💪

FAILURE: Build failed with an exception.

• What went wrong:
  Execution failed for task ':app:processReleaseGoogleServices'.

File google-services.json is missing. The Google Services Plugin cannot function without it.
Searched Location:
/Users/quintinwillison/code/ably/ably-flutter/example/android/app/src/release/google-services.json
/Users/quintinwillison/code/ably/ably-flutter/example/android/app/src/google-services.json
/Users/quintinwillison/code/ably/ably-flutter/example/android/app/src/Release/google-services.json
/Users/quintinwillison/code/ably/ably-flutter/example/android/app/google-services.json

• Try:
  Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

• Get more help at https://help.gradle.org/

BUILD FAILED in 56s
Running Gradle task 'assembleRelease'... 56.8s
Gradle task assembleRelease failed with exit code 1
{code}
@ikurek [responded (internal Slack message) that|https://ably-real-time.slack.com/archives/C0178QGC093/p1644237008457189?thread_ts=1644236687.002689&cid=C0178QGC093]:

I agree, but sadly there’s no easy way to solve this - as long as we embed Firebase into the project, this file will be required with each start. For now the only options I can see are:

• Include some sample google-services.json file attached to example Firebase instance (I don’t think it’s the best idea TBH)
• Create separate example for push notifications

I wonder if there's another approach, given how flexible Gradle is, where the build can detect if {{google-services.json}} is present and only enable the Google Services Plugin / Firebase complexities in that case. 🤔

[QuintinWillison]

@ikurek is going to research what we can do in terms of supplying a dummy/inert Google services JSON file in order to allow the example app to run without friction. For users who need to do PN testing with the app, we need to make sure there are instructions in the repo (contributing guide? PN guide?) that explain that they need to replace the dummy services file with their own, obtained from Firebase. Igor has also suggested we could also display some kind of disclaimer in the sample app to highlight that the runtime has detected that they're using a dummy GS file.