Add a flutter pub add action

[jakemac53]

Opening in case we are interesting in actually upstreaming this - I mostly just did it to get familiar with how actions are set up and plumbed through.

Using the action opens a dialog like this:

It does do some validation that a non-empty package name was input, but that is it today:

And it does ensure you actually clicked "OK" before actually running any commands.

[jwren]

LGTM

Revert out the gradle.properties changes. Also, you'll have to run ./bin/plugin generate to get the github presubmit action to pass.

[jakemac53]

Revert out the gradle.properties changes. Also, you'll have to run ./bin/plugin generate to get the github presubmit action to pass.

Done 👍