Add db migration for vscode_extension_marketplace

Issue: Create skeleton for Extensions Marketplace sett... (#508977 - closed)

What does this MR do and why?

Add db migration for vscode_extension_marketplace

• See this spike for the orig implementation
• This follows the guide to add jsonb column to app settings
• The setting can be empty, and "custom_values" is only required if "preset: 'custom'" is set

Changelog: changed

MR acceptance checklist

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Screenshots or screen recordings

Request to /api/v4/application/settings

Related to #508977 (closed)