Support publishing a plugin so that others can consume it (#7816).

A plugin consists of three pieces:

1. A plugin interface written in dart, which sends and receives HostMessages
2. Android specific code (java) which responds to HostMessages and interfaces with the native Android platform
3. iOS specific code (ojb-c or swift) which responds to HostMessages and interfaces with the native iOS platform

We assume that parts 2. and 3. is published on their existing distribution mechanisms, specifically mavencentral for java code and cocoapods for obj-c/swift code, and that some manifest is provided for declaring a dependency on those artifacts.

The functionality needed to actually build a plugin that depends on binaries in mavencentral/cocoapods is tracked in #7813.