feat: extra headers parameter in client options (#2091)

Closes #1106

Unfortunately, these need to be set at the connection level. I
investigated whether if we let users provide a callback they could use
`AsyncLocalStorage` to access their context. However, it doesn't seem
like NAPI supports this right now. I filed an issue:
https://github.com/napi-rs/napi-rs/issues/2456

docs/src/js/interfaces/ClientConfig.md

@@ -8,6 +8,14 @@
 
 ## Properties
 
+### extraHeaders?
+
+```ts
+optional extraHeaders: Record<string, string>;
+```
+
+***
+
 ### retryConfig?
 
 ```ts