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/src/__tests__/headerspec.test.ts

@@ -142,6 +142,7 @@ describe('HeaderSpecClient', () => {
     function mockFetch<T>(data: APIResponse<T>, ok = true) {
         return vi.fn().mockResolvedValue({
             ok,
+            headers: new Headers(),
             json: () => Promise.resolve(data),
         });
     }