Improve getGasInfo() type safety (#16)

- Change return type from Promise<any> to Promise<GasInfo>
- Provides better TypeScript type checking for gas pricing data
- Path already updated to /api/v1/network/gas in Issue #7
- GasInfo interface already defined in types.ts

Fixes #10

Author: umwelt

src/core/zhtp-api-methods.ts

@@ -595,8 +595,8 @@ export abstract class ZhtpApiMethods extends ZhtpApiCore {
     return this.request<any>('/api/v1/blockchain/status');
   }
 
-  async getGasInfo(): Promise<any> {
-    return this.request<any>('/api/v1/network/gas');
+  async getGasInfo(): Promise<GasInfo> {
+    return this.request<GasInfo>('/api/v1/network/gas');
   }
 
   async getNodeStatus(): Promise<NodeStatus> {