Fix app generate with Partners token

[gonzaloriestra]

WHY are these changes introduced?

Fixes https://github.com/shop/issues-develop/issues/21256

WHAT is this pull request doing?

• Use Destinations API to fetch beta flags to avoid permission issues

How to test your changes?

• CI
• SHOPIFY_CLI_PARTNERS_TOKEN=XXX p shopify app generate extension

Measuring impact

How do we know this change was effective? Please choose one:

• n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix
• Existing analytics will cater for this addition
• PR includes analytics changes to measure impact

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	78.54% (+0.01% 🔼)	13482/17165
🟡	Branches	72.56% (+0.04% 🔼)	6551/9029
🟡	Functions	78.8% (-0.01% 🔻)	3502/4444
🟡	Lines	78.9% (+0.01% 🔼)	12745/16153

Show new covered files 🐣

St.❔	File	Statements	Branches	Functions	Lines
🟢	... / beta-flags.ts	100%	100%	100%	100%

Test suite run success

3228 tests passing in 1358 suites.

Report generated by 🧪jest coverage report action from e3887b2