feat: convenience api for flags assets

[florentlotthepro]

Changes description

Add an API to get flags assets from a string.

Context

When we need to use a flag, we have a country code but currently, with the country code, we cannot get the flag.

Checklist

• [x] Check commits & PR names matches our requested structure. It must follow the https://www.conventionalcommits.org pattern.
• [x] Check your code additions will fail neither code linting checks.
• [x] I have reviewed the submitted code.
• [x] I have tested on a device/simulator.

Does this introduce a breaking change?

• No

Other information

How to use:

let frenchFlag: UIImage? = VitaminAssets.flag(countryCode: "fr")?.image

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