Serialize extra object with `json.Marshal`

[jeremybeard]

Description

This changes how the CLI serializes a connection's "extra" object (for use in the airflow connections add --conn-extra command) to use the standard library json.Marshal function, instead of hand-rolling JSON with string concatenation.

🎟 Issue(s)

Related #1608

🧪 Functional Testing

• Unit tests pass
• astro dev object import and astro dev object export work with no errors
• "extra" example from #1608 now imports correctly

📋 Checklist

• [x] Rebased from the main (or release if patching) branch (before testing)
• [x] Ran make test before taking out of draft
• [x] Ran make lint before taking out of draft
• [x] Added/updated applicable tests
• [ ] Tested against Astro-API (if necessary).
• [ ] Tested against Houston-API and Astronomer (if necessary).
• [ ] Communicated to/tagged owners of respective clients potentially impacted by these changes.
• [ ] Updated any related documentation

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 70.37037% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 86.55%. Comparing base (f35749c) to head (d9f658f). Report is 1 commits behind head on main.

Files	Patch %	Lines
settings/settings.go	70.37%	5 Missing and 3 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1610 +/- ## ========================================== - Coverage 86.59% 86.55% -0.04% ========================================== Files 114 114 Lines 16080 16084 +4 ========================================== - Hits 13924 13922 -2 - Misses 1293 1297 +4 - Partials 863 865 +2 ```

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