[cloud_functions] Update cloud_functions to use platform interface

[sbeitzel]

Description

This change updates the existing cloud_functions plugin to use cloud_functions_platform_interface.

Related Issues

This addresses issue flutter/flutter#45299
It's also a drive-by fix for issue #95

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Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• No, this is not a breaking change.

[harryterkelsen]

LGTM with nits

[KevinTheGray]

@sbeitzel could you give me a quick rundown on/if there's anything to do for the following issue I had when upgrading to this version (this is on an iOS simulator):

I haven't dug too deeply, and just downgraded for now as I have to get a quick build out.

this is the call I was making

HttpsCallableResult callableResult = await CloudFunctions.instance
          .getHttpsCallable(
            functionName: functionName,
          )
          .call(parameters);

I can dig a little deeper later today if this is an issue on my end.

[sbeitzel]

I think it was an oversight on my end. Looking at the MethodChannel being registered in the new version versus the old version, I think I copy/pasted from the wrong place. The error, I suspect, lies in method_channel_cloud_functions.dart -- in the platform interface library rather than the cloud_functions library. I have to run to a meeting right now but I will prepare a patch later today.

[KevinTheGray]

Sounds good, thanks!

[sbeitzel]

Here's the PR to fix the problem.

[cacianokroth]

Why does this catch remain, if an error does not fall into it?