feat(headers): add support for custom HTTP headers in clients

* Introduced `headers` property in `ClientConfig` interface.
* Updated `HeaderSpecClient` and `ResolveSpecClient` to utilize custom headers.
* Implemented `mergeHeaders` function to handle case-insensitive header merging.
* Added tests for custom header functionality in clients.

resolvespec-js/dist/index.d.ts

@@ -34,6 +34,8 @@ export declare function buildHeaders(options: Options): Record<string, string>;
 export declare interface ClientConfig {
     baseUrl: string;
     token?: string;
+    /** Custom HTTP headers. Token and HeaderSpec query options take precedence. */
+    headers?: Record<string, string>;
 }
 
 export declare interface Column {