Add user_scan feature for mix.com user profiles

[KagemaNjoroge]

This pull request adds a new validation function for the Mix social platform to the user scanning module. The function checks if a username is available or taken on Mix by making an HTTP request and interpreting the response.

New social platform validation:

• Added a new validate_mix function in user_scanner/user_scan/social/mix.py that checks username availability on Mix, using a random user agent and a generic validation helper.

Recon findings

• Manual curl testing was performed with one known existing username and one guaranteed non-existent username.
• Existing usernames consistently returned HTTP 200.
• Non-existent usernames consistently returned HTTP 404.
• No redirects affecting reliability were observed in these checks.
• No CAPTCHAs, Cloudflare challenges, or aggressive anti-bot blocking were encountered.

Verification

• Confirmed consistent behavior across repeated tests.
• Implementation follows the existing module structure and import style used in the social category.
  Resolves [New Module] Add username support for mix.py #306

[kaifcodec]

@KagemaNjoroge Thank you for the contribution!
I tested it locally and looks good to me.
If you want to contribute further you are welcome to add new sites.