AWS Amplify Support for ap-southeast-4 (Melbourne) Region

### Before opening, please confirm:

- [x] I have checked to see if my question is addressed in the [FAQ](https://github.com/aws-amplify/amplify-hosting/blob/master/FAQ.md).
- [x] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-hosting/issues?q=is%3Aissue+).
- [x] I have removed any sensitive information from my code snippets and submission.

### Amplify Hosting feature

Not Applicable

### Is your feature request related to a problem? Please describe:

Currently forced to host Amplify frontend in ap-southeast-2 (Sydney) while backend services (API Gateway, Lambda, Cognito, RDS) run in ap-southeast-4 (Melbourne).

This creates:

- 15-25ms additional latency per API request
- Poor user experience for Melbourne-based customers
- Increased data transfer costs for cross-region communication
- Suboptimal architecture requiring workarounds

**Use Case:**

- Australian enterprise application serving users primarily in Victoria/Melbourne region. Need co-location of frontend and backend services for optimal performance.

**Workaround or temp solution**

Use CloudFront + S3 instead of Amplify

- Host static files in S3 (ap-southeast-4)
- CloudFront edge locations serve content globally
- Backend stays in ap-southeast-4

### Describe how you'd like this feature to work

**Deployment:**

- Select ap-southeast-4 (Melbourne) from region dropdown in Amplify Console
- Connect GitHub/GitLab repository as usual
- Automatic builds and deployments work identically to other regions

**Infrastructure:**

- Amplify hosting infrastructure provisioned in Melbourne data centers
- CDN edge locations remain global (existing CloudFront behavior)
- Build servers located in ap-southeast-4 for faster build times

**Integration:**

Seamless connection to other AWS services in ap-southeast-4:

1. API Gateway endpoints
2. Cognito user pools
3. Lambda functions
4. RDS databases
5. Environment variables and backend configurations work without cross-region calls

**Performance:**

- Sub-5ms latency to backend services (same region)
- Faster build times due to local build infrastructure
- Reduced data transfer costs

**Management:**

Same Amplify Console experience
Identical CLI commands and workflows
Branch-based deployments and previews
Custom domains and SSL certificates

- Exactly like current Amplify functionality but with Melbourne as deployment region option, eliminating cross-region latency for Australian applications.

Start by reviewing how Amplify Hosting represents deployment regions and provisions hosting infrastructure, then compare the existing ap-southeast-2 workflow with the requested ap-southeast-4 support. Done means Melbourne is selectable in the Amplify Console, builds and deployments work there, and integrations with regional AWS services avoid the stated cross-region workaround.

Written by the indexing model from the issue text.