consolidate `labs` packages into this repo

[devoncarew]

We'd like to consolidate individual packages into monorepo as per dart-lang/sdk#56591. As part of that work, we'd like to consolidate labs packages into this repo. Many of the labs packages are already in topic monorepos (http, ...), so we'll only worry about labs packages that are not already in other monorepos.

As labs packages mature, they can be moved out of this repo into other monorepos (core, tools, topic monorepos). If we decide to discontinue a package, it can be archived in place (the code deleted - but still available in the commit history - and readme and other docs + issues updated appropriately).

Potential repos to move here:

• dart-lang/appengine transfer package:appengine #98
• dart-lang/gcloud transfer package:gcloud #14
• dart-lang/native_synchronization transfer package:native_synchronization #3

[devoncarew]

cc @kevmoo @jonasfj @sigurdm and @isoos (people who have interest in the above appengine and gcloud repos)