Improve rendering of `Owners` in `safe inspect --json`

Currently `safe inspect --json` returns the Safe `Owners` value as a comma-separated string, whereas a more JSON-native solution would render this as a JSON list of addresses. In addition, the number of owners should be moved to in a separate field, as it currently unnecessarily difficult to match the `Owners` key programmatically due to it appearing with a count, ie. `Owners(5)`.