[cloud_functions] web plugin implementation

[sbeitzel]

Description

This PR delivers the web implementation of the cloud_functions plugin. Note that although README.md refers to the example in cloud_functions, that can't actually be updated until this plugin gets published once (otherwise version resolving will fail).

Related Issues

Resolves issue 45299

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• If the pull request affects only one plugin, the PR title starts with the name of the plugin in brackets (e.g. [cloud_firestore])
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See [Contributor Guide]).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the [Flutter Style Guide].
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy].
• I updated CHANGELOG.md to add a description of the change.
• I signed the [CLA].
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

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

• No, this is not a breaking change.

[sbeitzel]

It looks like the failure in build-ipas+drive-examples CHANNEL:master PLUGIN_SHARDING:--shardIndex 1 --shardCount 4 is not because of the plugin code but because that build check sets up for iOS 13.3 and then tries to run a simulator with iOS 13.2, which hasn't been installed. Who's in charge of configuring the check scripts?

[harryterkelsen]

LGTM with a few documentation updates

[flowhorn]

Is there support for other regions? Because in the used firebase package there is no support right now and the region parameter isn't used I think. Or am I wrong?

[sbeitzel]

This implementation delegates to the firebase plugin and you're right, the region parameter is silently ignored because there's not an obvious way to pass it down.

[harryterkelsen]

You can set the region by doing firebase.app().functions('region-name'): https://github.com/FirebaseExtended/firebase-dart/blob/master/lib/src/app.dart#L58

[harryterkelsen]

LGTM. @collinjackson can you take a look?

[collinjackson]

LGTM