eval: Add support for reading from a .env file (#29426)

This PR adds support for the eval to read environment variables from a
`.env` file located in the `crates/eval` directory.

For instance, you can use it to set your Anthropic API key:

```
ANTHROPIC_API_KEY=<secret>
```

Release Notes:

- N/A

crates/eval/Cargo.toml

@@ -19,6 +19,7 @@ client.workspace = true
 collections.workspace = true
 context_server.workspace = true
 dirs = "5.0"
+dotenv.workspace = true
 env_logger.workspace = true
 extension.workspace = true
 fs.workspace = true