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

Cargo.lock

@@ -4548,6 +4548,12 @@ dependencies = [
  "syn 2.0.100",
 ]
 
+[[package]]
+name = "dotenv"
+version = "0.15.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "77c90badedccf4105eca100756a0b1289e191f6fcbdadd3cee1d2f614f97da8f"
+
 [[package]]
 name = "dotenvy"
 version = "0.15.7"
@@ -4970,6 +4976,7 @@ dependencies = [
  "collections",
  "context_server",
  "dirs 5.0.1",
+ "dotenv",
  "env_logger 0.11.8",
  "extension",
  "fs",