[next]: Add data fetching and preview

[ahuseyn]

Description

This PR adds following:

• data-fetching package. This is an initial phase of the package. Could be merged with template-hierarchy or graphql in the future or stand as is.
• enablePreview utility for the nextjs adapter. It gives a customizable API handler to enable the Draft Mode.
• kitchen-sink example for Nextjs to include all the available features (template-hierarchy, previews, data-fetching). It'll be the working ground for future development.

and updates following:

• graphql client package to fix some errors and allow custom headers for auth
• templateHierarchy.js to add seedNode property to templateData. We should remove seedQuery in favor of seedNode, as it better represents the value assigned to it.
• seedQueryExecutor.js to add databaseId and asPreview parameters

Related Issue(s):

• [experimental]: PoC Previews + Nextjs Implementation #2151
• [experimental]: GraphQL Data Fetching Support #2150

[josephfusco]

Great work @ahuseyn!

I left some non-blocking feedback to consider.

[josephfusco]

Consider using Promise.allSettled instead of Promise.all

This would allow successful queries to return even if one fails.

[ahuseyn]

Thanks Joe, my initial motivation to use Promise.all was this comment, but we should definitely reevaluate that approach in new Faust.