Support altering the default script for AzurePostgresFlexibleServer in .NET Aspire

### Is there an existing issue for this?

- [x] I have searched the existing issues

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

Currently, the `AddAzurePostgresFlexibleServer(...).AddDatabase(...).WithCreationScript(...)` API works well, but it returns an `IResourceBuilder` which is not compatible with `PostgresDatabaseResource`. This inconsistency makes it difficult to generalize database setup logic (e.g., if switching between local Postgres and Azure).

**Screenshot of the error for reference:**



### Describe the solution you'd like

It would be helpful if:

- **There was a unified interface for database resources.**
- **Or better guidance/docs/examples for using `WithCreationScript` in the `AzurePostgresFlexibleServer` scenario.**

### Additional context

_No response_