Misc refactoring ahead of #2390 / !1957

• Extracted from !1957 (merged)
• Related to #2390 (closed)

Merge Request Checklist

• Link to an issue
• Include the proposed fix or feature
• Include and update tests for the modified code
  • Fix tests
  • Add new tests
    • Let Canonical.update take kwargs so that attrs can look like init args
    • Add Canonical.with_attrs to create copy with changes
    • Let default values be specified on Canonical
    • Let Canonical fall back on other object for unknown and specified known attributes
    • Move env var expansion from SettingsStore to SettingsService
    • Add ProjectPlugin.config_with_extras property composed of .config and .extras

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