Support RDS IAM authentication metadata in configure({ database })

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

Generated SQL schemas currently use `configure({ database })` to store database connection metadata:

```ts
export const schema = configure({
database: {
identifier: "IDYufuVoPi8Po1obyjhhITYQ",
engine: "postgresql",
connectionUri: secret("SQL_CONNECTION_STRING")
}
}).schema({
// models...
});
```

This works for password-based connection URIs such as:

```txt
postgres://user:password@hostname:port/database
```

However, there is currently no explicit way to represent a database connection that uses **RDS IAM database authentication** instead of a static password.

This is needed for use cases such as Aurora PostgreSQL Express configuration, where the connection may rely on IAM auth tokens rather than a password in the connection URI.

**Describe the solution you'd like**

I would like `configure({ database })` to support an optional authentication strategy field.

For example:

```ts
export const schema = configure({
database: {
identifier: "IDYufuVoPi8Po1obyjhhITYQ",
engine: "postgresql",
connectionUri: secret("SQL_CONNECTION_STRING"),
authentication: {
strategy: "rdsIam"
}
}
}).schema({
// models...
});
```

When `authentication` is omitted, the current behavior should remain unchanged and default to password-based authentication.

When `authentication.strategy === "rdsIam"`, downstream packages should be able to understand that:

- the database uses RDS IAM authentication;
- `connectionUri` may omit the password;
- `connectionUri` still provides host, port, database name, and username.

This issue is only about allowing `@aws-amplify/data-schema` to express this metadata. Runtime support, such as generating IAM auth tokens and configuring IAM permissions, would be handled in downstream packages.

**Describe alternatives you've considered**

One alternative is to encode the auth mode in the URI, for example:

```txt
postgres+iam://postgres@hostname:5432/database
```

However, an explicit typed field is clearer and easier for downstream packages (https://github.com/aws-amplify/amplify-category-api) to consume.

Another alternative is to manually edit generated `schema.sql.ts` files, but those files are autogenerated and would be overwritten.

**Additional context**

This is motivated by supporting Aurora PostgreSQL Express configuration and other RDS PostgreSQL setups that use IAM database authentication.

The goal is to extend the existing database metadata shape in a backward-compatible way:

```ts
configure({
database: {
identifier,
engine,
connectionUri,
vpcConfig,
authentication // optional
}
})
```

A companion issue may be needed in `amplify-category-api` or other backend/runtime packages to consume this metadata and implement the actual RDS IAM authentication behavior.

Start at the configure({ database }) type and metadata path in @aws-amplify/data-schema; generated schema.sql.ts files should not be edited. Confirm how database metadata is represented and consumed, then add the optional authentication.strategy value while preserving password-based behavior when it is omitted. Done means downstream packages can distinguish rdsIam and receive the host, port, database name, and username without requiring a password.

Written by the indexing model from the issue text.