feat(gtm): add onBeforeGtmStart

[huang-julien]

🔗 Linked issue

resolve #243

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This PR improves GTM by adding a new callback to allow users to push anything into the datalayer before we push gtm.start . This way, users will be able to correctly set the consent mode

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Name	Status	Preview	Comments	Updated (UTC)
scripts-docs	❌ Failed (Inspect)			Feb 17, 2025 9:51pm
scripts-playground	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Feb 17, 2025 9:51pm

[harlan-zw]

This is a great escape hatch. Would it be possible to document with an example of when you'd use this 🙏

I think if all EU users need the consent mode we should have a dedicated docs section for that 🤔

[huang-julien]

Seems like we DO need to use a gtag() fn in order to configure the consent mode.... This makes 0 sense.

[huang-julien]

I think if all EU users need the consent mode we should have a dedicated docs section for that 🤔

Are you talking about a whole page ?

[harlan-zw]

Nah just like a section in the GTM page, let's go with this for now. Thanks!