feat(tanstack-start): Add getAuth server function

[octoper]

Description

This PR introduces getAuth server function that can be used inside the TanStack start createServerFn.

Example:

import { getAuth } from '@clerk/tanstack-start/server'
import { createServerFn } from '@tanstack/start'


export const fetchCurrentUserPosts = createServerFn('GET', async (_payload, ctx) => {
  const { userId } = await getAuth(ctx)

  if (!userId) {
    ...
  }

  ...
})

Checklist

• npm test runs as expected.
• npm run build runs as expected.
• (If applicable) JSDoc comments have been added or updated for any package exports
• (If applicable) Documentation has been updated

Type of change

• 🐛 Bug fix
• 🌟 New feature
• 🔨 Breaking change
• 📖 Refactoring / dependency upgrade / documentation
• other:

[changeset-bot]

🦋 Changeset detected

Latest commit: 1afafcd

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@clerk/tanstack-start	Minor

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[wobsoriano]

Looking good 🚀