feat(nextjs): Support Keyless mode (#4602)

Author: panteliselef

.changeset/dry-cats-change.md

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+---
+'@clerk/backend': minor
+---
+
+New **experimental** API: `AccountlessApplicationAPI`
+
+Inside `clerkClient` you can activate this new API through `__experimental_accountlessApplications`. It allows you to generate an "accountless" application and the API returns the publishable key, secret key, and an URL as a response. The URL allows a user to claim this application with their account. Hence the name "accountless" because in its initial state the application is not attached to any account yet.

.changeset/grumpy-camels-think.md

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+---
+'@clerk/clerk-react': minor
+---
+
+Various internal changes have been made to support a new feature called "Keyless mode". You'll be able to use this feature with Next.js and `@clerk/nextjs` initially. Read the `@clerk/nextjs` changelog to learn more.
+
+List of changes:
+- A new internal prop called `__internal_bypassMissingPublishableKey` has been added. Normally an error is thrown when the publishable key is missing, this disables this behavior.
+- Loading of `clerk-js` won't be attempted when a missing key is present
+- A new instance of `IsomorphicClerk` (an internal Clerk class) is created for each new publishable key