Add easier github loading of plugins

[jmetz]

As there is a recommended way of using github repos to host plugins (https://imjoy.io/docs/#/development?id=hosting-and-deploying-plugins), seems like it would be nice to go one step further similar to many modern plugin systems that heavily use github, for loading all plugins hosted at a repo.

Proposal

Add convenience function loadGithubPlugins (or similar), that loads a github repo's manifest, and loads all plugins listed there.

[jmetz]

As mentioned here : bioimage-io/spec-bioimage-io#550 (comment) this feature exists at least partially.

Updating now to reflect loading a collection of plugins in the same way.