feat: add polar provider

[ahmedrangel]

As requested, this PR closes #173

For testing follow these steps:

1. Sign up at https://polar.sh/signup
2. Go to account settings https://polar.sh/settings
3. Look for OAuth Applications setting
4. Click New OAuth App
5. Fill your Application Name
6. In Client Type, select Confidential Client
7. In Redirect URIs, add http://127.0.0.1:3000/auth/polar
8. In Scopes, check openid and email (It's important to check these scopes for fetching user info, otherwise, the endpoint will return error 500)
9. In Homepage URL, write any URL
10. Click Create
11. Copy your Client ID and Client Secret
12. Add to .env NUXT_OAUTH_POLAR_CLIENT_ID and NUXT_OAUTH_POLAR_CLIENT_SECRET
13. Test polar auth

[pkg-pr-new]

Open in Stackblitz

pnpm add https://pkg.pr.new/atinux/nuxt-auth-utils@196

commit: eabe0f5

[atinux]

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