[firebase_remote_config] Add macOS support

[davcannon]

Description

Adds macOS support to the firebase_remote_config plugin

Related Issues

Fixes #2697, also related to #1653

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[kroikie]

@balconywood thanks for the PR! Before we can support macos we should move the Remote Config Plugin to platform interface. I would be happy to review a PR that does that then look at adding different platform support. Would you be interested in providing a platform interface implementation for Remote Config?

[davcannon]

@kroikie Sure, I'll see what I can do

[Salakar]

MacOS support for Remote Config has landed on master from another PR of mine and a pre-release is available, so I'm going to go ahead and close this PR, sorry 🙈 thanks for contributing though!

https://firebase.flutter.dev/docs/remote-config/overview