feat: Add CognitoUserPoolDomain component

[wcvo]

Adds support for configuring Cognito User Pool domains via domain prop on CognitoUserPool. This addresses the feature request in #5220.

Features:

• Cognito prefix domains (e.g., my-app.auth.us-east-1.amazoncognito.com)
• Custom domains with automatic ACM certificate creation and DNS configuration
• Multiple DNS providers (Route 53, Cloudflare, Vercel)
• Option to use existing certificates

Usage:

// Prefix domain
new sst.aws.CognitoUserPool("MyUserPool", {
   domain: {
      prefix: "my-app-dev"
   }
});

// Custom domain with Route 53
new sst.aws.CognitoUserPool("MyUserPool", {
   domain: "auth.example.com"
});

// Custom domain with Cloudflare
new sst.aws.CognitoUserPool("MyUserPool", {
   domain: "auth.example.com"
   dns: sst.cloudflare.dns()
});

Test plan

• Tested prefix domain creation
• Tested custom domain creation
• Tested domain change (deleteBeforeReplace)

[jamesgibbons92]

Good contribution @wcvo !
Is there a reason you chose a method addDomain(), over just adding a domain prop to the user pool component? I think a prop makes more sense and fits the pattern used in other sst components which support custom domains

[wcvo]

@jamesgibbons92 Valid point! I initially went with a method to match the existing addClient() and addIdentityProvider() patterns in the component, but you're right that a prop makes more sense since a User Pool can only have one domain (vs multiple clients/providers). Will update shortly.

[dimitor115]

Nice work! I’ve been looking for this 😄 If I may suggest: since this is such a convenient setup, it’d be great to include the domain usage in the aws-cognito example as well. It should render correctly once my other PR is merged.

[vimtor]

let's wait on merging #6271 first