feat: add support for custom verification token generation

[ochrstn]

What?

This PR adds the ability for developers to define custom verification token generation logic in PayloadCMS authentication. It introduces a new optional generateVerificationToken function that can be configured in the auth.verify settings.

Why?

Currently, verification tokens are generated using a hardcoded crypto.randomBytes(20).toString('hex')approach. This limitation prevents developers from:

• Implementing custom token formats or domain-specific token patterns
• Integrating with external token services
• Meeting specific security or compliance requirements

How?

Check for custom token generator. Supports both synchronous and asynchronous custom token generators. Maintains backward compatibility by falling back to default crypto.randomBytes(20).toString('hex').

Usage Example:

export const Users: CollectionConfig = {
  slug: 'users',
  auth: {
    verify: {
      generateVerificationToken: ({ req, user }) => {
        return `custom-${user.id}-${Date.now()}`
      },
    },
  },
  // ...
}