Resolve "Allow plugins to inherit configuration from others"

Merge Request Checklist

• Link to an issue
• Include the proposed fix or feature
  • Refactor plugins around inheritance
    • Misc: !1971 (merged)
    • Plugins: !1972 (merged)
  • Implement inherit_from
  • Add --inherit-from flag to meltano add (and --as?)
  • Meltano UI: Configure existing inherited plugins
  • Set ProjectPlugin.parent lazily in ProjectPluginsService to improve performance
  • Raise special error when --inherit-from on meltano add or inherit_from in meltano.yml doesn't exist
  • Optional
    • Add meltano list to list "inherits from" relations
    • Only install plugins once per pip_url
    • Meltano UI: Show inherited settings differently
    • Meltano UI: Add the same discoverable plugin multiple times using inheritance
    • Meltano UI: Add a new plugin by inheriting from an existing one
    • Have meltano add suggest adding --custom when plugin was not known
• Include and update tests for the modified code
  • Make existing tests pass
  • Write tests for new functionality
    • Let ProjectPlugin inherit from another ProjectPlugin or BasePlugin with a different name
    • Let plugin inherit configuration of parent plugin
    • Don't read settings from custom env vars when plugin name doesn't match parent
    • Use ProjectPluginsService.get_parent in ProjectAddService.add for consistency
    • Add meltano add --inherit-from and --as options
• Include a documentation change: !1984 (merged)

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Implements #2390 (closed)