Customize Backpack asset through options

[moniika]

Category Plugins

Component @blockly/workspace-backpack

Is your feature request related to a problem? Please describe. Changing the button asset for the Backpack is expected to be a common use case, but doing so requires very involved subclassing of the Backpack class.

Describe the solution you'd like Support customizing Backpack asset used through options passed in when the Backpack is created.

Describe alternatives you've considered Use method on the Backpack object rather than passing in as an option.

Additional context https://github.com/google/blockly-samples/issues/808