Add support for code snippet 'repositories'

[ptitzler]

Is your feature request related to a problem? Please describe. Users can currently add code snippets by creating them manually or by importing them from the local file system using the elyra-metadata import CLI command. It would be nice if it was possible to pull in code snippets from remote sources, such as https://github.com/dvillaj/pandas-code-snippets.

Describe the solution you'd like Potential ideas, some of which were briefly discussed in today's dev meeting:

• Support loading of code snippets from remote repositories, similar to what is currently supported for custom components
• Support importing code snippets from remote repositories. This would in essence download code snippet metadata from remote sources and make them permanently available in the local installation.

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered.

Additional context Add any other context or screenshots about the feature request here.