Optional headers for wasm

## Feature request

It would be nice to have a check box to enable wasm headers. I am not sure if this is particular for flutter, which suggests https://docs.flutter.dev/platform-integration/web/wasm#serve-the-output-with-an-http-server

### Feature description

Create a check box that adds a block of custom directive with required headers, something like:

```
location / {
add_header Cross-Origin-Embedder-Policy "credentialless"
add_header Cross-Origin-Opener-Policy "same-origin"
}
```

### How the feature is useful

Enables use of wasm when correct headers are sent

The issue names no repository files or tests. Start by tracing the option controls and generated NGINX template, then verify that enabling the checkbox emits the two requested headers in the location block while leaving output unchanged when disabled.

Written by the indexing model from the issue text.