Feature Proposal: WithEnvFile for Aspire resources

## Background and Motivation

Enable Aspire resources to load environment variables from `.env` files and map them to Aspire parameters. This simplifies configuration and supports best practices for parameter management and secrets.

## Proposed API

```csharp
public static IResourceBuilder WithEnvFile(this IResourceBuilder builder)
where T : IResourceWithEnvironment;

public static IResourceBuilder WithEnvFile(
this IResourceBuilder builder,
Action> configure)
where T : IResourceWithEnvironment;
```

### EnvEntry Structure (Immutable)

```csharp
public sealed class EnvEntry
{
public string Key { get; }
public string? Value { get; }
public string? Comment { get; }

public EnvEntry(string key, string? value, string? comment)
{
Key = key;
Value = value;
Comment = comment;
}
}
```

## Mapping Rules

- By default, each `.env` entry is mapped to a non-secret parameter with the key and value.
- Comments from the `.env` file are available via the `Comment` property on `EnvEntry`.
- The second overload allows the user to customize:
- Whether a parameter is a secret (based on key name or custom logic)
- The parameter description or other metadata
- Skipping or modifying entries before mapping
- **Parameter Name Uniqueness:**
To ensure uniqueness for parameters originating from an `.env` file, the naming scheme should be `{resource}-env-{key}` (where `{resource}` is the logical name or ID of the resource and `{key}` is the environment variable key, lowercased). This avoids collisions across resources and makes the origin clear.
- Example: For resource `"ui"` and key `"API_KEY"`, the parameter name would be `"ui-env-api_key"`.

### Usage Example

```csharp
builder.AddNpmApp("ui", "../frontend")
.WithEnvFile();

builder.AddNpmApp("ui", "../frontend")
.WithEnvFile((entry, paramBuilder) =>
{
if (entry.Key.EndsWith("_SECRET"))
paramBuilder.Resource.Secret = true;
paramBuilder.WithDescription(entry.Comment ?? $"Imported from .env: {entry.Key}");
// Parameter name will automatically be "ui-env-{key.ToLowerInvariant()}"
});
```

## Risks & Considerations

- **File not found:** If `.env` is not found, nothing happens.
- **Duplicate keys:** If duplicate keys exist, only the last one is used.
- **Sensitive data:** Take care not to accidentally expose secrets.
- **Error handling:** Consider adding logging or errors for malformed lines.
- **Customization:** Users have full control via the callback to decide how each env entry is mapped.
- **Parameter Naming:** Uniqueness is guaranteed by the `{resource}-env-{key}` (lowercased) scheme.
- **Immutability:** `EnvEntry` is immutable for thread safety and reliability.

---

Start by reviewing Aspire's existing IResourceBuilder, IResourceWithEnvironment, and ParameterResource APIs against the proposed WithEnvFile overloads and EnvEntry shape. Define completion around parsing .env entries, applying the resource-qualified parameter names, supporting callback customization, and covering the stated missing-file, duplicate-key, malformed-line, comment, and secret-handling behaviors.

Written by the indexing model from the issue text.