Expand Network Metrics API to Support Epoch Parameter

## Description
Enhance the **Network Metrics API** to support fetching metrics for **any given Cardano epoch** as a parameter. Currently, the API only returns data for the **latest epoch**.

Queries for reference - https://github.com/IntersectMBO/govtool/blob/develop/govtool/backend/sql/get-network-metrics.sql
Haskell backend API - https://github.com/IntersectMBO/govtool/blob/develop/govtool/backend/src/VVA/Network.hs
Typescript backend API - https://github.com/IntersectMBO/govtool-assembly/blob/develop/backend/src/network/network.controller.ts

## Acceptance Criteria
- [ ] The **Network Metrics API** accepts an **epoch parameter** to request metrics for a specified epoch.
- [ ] If no epoch parameter is provided, the API defaults to returning metrics for the **latest epoch**.
- [ ] The API correctly returns metrics for past epochs when a valid parameter is provided.