Add getEntryPointContractInstance function in service-manager

## Background
Query on EntryPoint contract needs to load its contract instance from EPAddress, Ep-ABI and provider on a given chainId

## Change Proposal
Add a function `getEntryPointContractInstance` in `service-manager.ts`
maintain a map with key as networkId and value as EPContractInstance

```js
const epContractInstanceMap = new Map();
```

Start in service-manager.ts and trace how EntryPoint contract queries currently obtain their provider, address, ABI, and chainId. Implement the requested getEntryPointContractInstance entry point and verify that the per-network instance map is reused for subsequent queries; the issue does not name a test file or command.

Written by the indexing model from the issue text.