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/common/http.ts

@@ -0,0 +1,30 @@
+import type { ClientConfig } from './types';
+
+/** Merge HTTP headers case-insensitively, preserving the winning spelling. */
+export function mergeHeaders(...sources: Record<string, string>[]): Record<string, string> {
+    const result: Record<string, string> = {};
+    for (const source of sources) {
+        for (const [name, value] of Object.entries(source)) {
+            for (const existing of Object.keys(result)) {
+                if (existing.toLowerCase() === name.toLowerCase()) delete result[existing];
+            }
+            Object.defineProperty(result, name, { value, enumerable: true, configurable: true, writable: true });
+        }
+    }
+    return result;
+}
+
+export function clientHeaders(config: ClientConfig): Record<string, string> {
+    return mergeHeaders(
+        { 'Content-Type': 'application/json' },
+        config.headers ?? {},
+        config.token ? { Authorization: `Bearer ${config.token}` } : {},
+    );
+}
+
+export function clientCacheKey(config: ClientConfig): string {
+    const headers = Object.entries(clientHeaders(config))
+        .map(([name, value]) => [name.toLowerCase(), value])
+        .sort(([a], [b]) => a.localeCompare(b));
+    return JSON.stringify([config.baseUrl, headers]);
+}