fix(microsoft-entra-id): Support corporate proxies with customFetch

[frendsick]

☕️ Reasoning

The library supports corporate proxies using a custom fetch function with the customFetch property. However, the MicrosoftEntraID provider cannot be used from behind a corporate proxy because the built-in fetch function is used unconditionally within the provider instead. The customFetch function should always be used in place of the built-in fetch when it is defined.

This pull request patches the MicrosoftEntraID provider always to use the customFetch function if it is defined, allowing it to be used from behind corporate proxies.

🧢 Checklist

• Documentation
• Tests
• Ready to be merged

🎫 Related issues

Relates: #12291

📌 Resources

• Supporting corporate proxies
• Microsoft Entra ID provider

🩹 Other possibly affected providers

The built-in fetch function is also used in many other providers. Based on its usage, the following providers could be affected by the same issue. I have not tested the hypothesis.

>> grep -rl " fetch(" next-auth/packages/core/src/providers/
packages/core/src/providers/tiktok.ts
packages/core/src/providers/vk.ts
packages/core/src/providers/github.ts
packages/core/src/providers/microsoft-entra-id.ts
packages/core/src/providers/credentials.ts
packages/core/src/providers/dropbox.ts
packages/core/src/providers/apple.ts
packages/core/src/providers/resend.ts
packages/core/src/providers/azure-devops.ts
packages/core/src/providers/loops.ts
packages/core/src/providers/sendgrid.ts
packages/core/src/providers/nextcloud.ts
packages/core/src/providers/todoist.ts
packages/core/src/providers/foursquare.ts
packages/core/src/providers/wechat.ts
packages/core/src/providers/postmark.ts
packages/core/src/providers/hubspot.ts
packages/core/src/providers/mailgun.ts
packages/core/src/providers/facebook.ts
packages/core/src/providers/forwardemail.ts
packages/core/src/providers/notion.ts
packages/core/src/providers/trakt.ts

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