feat: support setting headers via env

Author: stainless-app[bot]

src/client.ts

@@ -241,6 +241,18 @@ export class Finch {
     this.fetch = options.fetch ?? Shims.getDefaultFetch();
     this.#encoder = Opts.FallbackEncoder;
 
+    const customHeadersEnv = readEnv('FINCH_CUSTOM_HEADERS');
+    if (customHeadersEnv) {
+      const parsed: Record<string, string> = {};
+      for (const line of customHeadersEnv.split('\n')) {
+        const colon = line.indexOf(':');
+        if (colon >= 0) {
+          parsed[line.substring(0, colon).trim()] = line.substring(colon + 1).trim();
+        }
+      }
+      options.defaultHeaders = { ...parsed, ...options.defaultHeaders };
+    }
+
     this._options = options;
 
     this.accessToken = accessToken;