Support specific external connection string

### Is there an existing issue for this?

- [x] I have searched the existing issues

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

Aspire currently provides `AddConnectionString`, but it is a generic abstraction that requires consumers to either:

- provide a fully-formed connection string, or
- manually build the connection string using `ReferenceExpression`s.

This creates unnecessary duplication because resource-specific hosting packages such as `Aspire.Hosting.SqlServer`, `Aspire.Hosting.PostgreSQL`, and `Aspire.Hosting.Redis` already contain the logic required to generate provider-specific connection strings in their supported formats (ADO.NET, URI, JDBC, etc.).

As a result, users cannot easily leverage that existing logic when defining standalone `ConnectionStringResource`s, and they lose access to resource-specific metadata and integrations.

### Describe the solution you'd like

Introduce provider-specific helpers for creating connection string resources, for example:

```csharp
IResourceBuilder AddSqlServerConnectionString(
string name,
EndpointReference host,
ReferenceExpression username,
ReferenceExpression password);

IResourceBuilder AddPostgresSqlConnectionString(
string name,
EndpointReference host,
ReferenceExpression username,
ReferenceExpression password);

IResourceBuilder AddRedisConnectionString(
string name,
EndpointReference host,
ReferenceExpression password);
```

These APIs would:

- Reuse the same connection string generation logic already implemented by the corresponding Aspire hosting packages.
- Support provider-specific formats where applicable (ADO.NET, URI, JDBC, etc.).- Integrate with `GetConnectionProperties()`.
- Enable provider-specific health checks and metadata.
- Reduce duplication and improve consistency between resource definitions and generated connection strings.

Start by comparing AddConnectionString with the provider-specific logic in Aspire.Hosting.SqlServer, Aspire.Hosting.PostgreSQL, and Aspire.Hosting.Redis. Trace how GetConnectionProperties() and existing connection-string generation are exposed; done means the proposed provider-specific helpers support the listed formats, metadata, integrations, and health checks consistently.

Written by the indexing model from the issue text.