Add support for Cloud Tasks

[bryanoltman]

Adds minimal Cloud Tasks support (creating a task with an http request).

I'm not sure if this is built out enough to warrant merging, and I imagine documentation updates would be required as well. lmk what you think!

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[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:appengine	0.13.11	already published at pub.dev
package:gcloud	0.10.0	ready to publish	gcloud-v0.10.0
package:googleapis_firestore_v1	0.1.0-wip	WIP (no publish necessary)
package:native_synchronization	0.3.1	already published at pub.dev

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