Decouple net from xrpld and move rpc related classes to the rpc folder

[a1q123456]

High Level Overview of Change

As a step of modularisation, we're moving things from xrpld to libxrpl. This PR moves net and separates it with rpc.

Context of Change

This is a refactor and it's a step of modularisation.

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Refactor (non-breaking change that only restructures code)
• [ ] Performance (increase or change in throughput and/or latency)
• [ ] Tests (you added tests for code that already exists, or your new feature included in this PR)
• [ ] Documentation update
• [ ] Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
• [ ] Release

API Impact

This PR doesn't affect API

• [ ] Public API: New feature (new methods and/or new fields)
• [ ] Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• [ ] libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• [ ] Peer protocol change (must be backward compatible or bump the peer protocol version)

Test Plan

This is a trivial PR and it shouldn't affect existing functionalities. We can do some regression tests for confirmation.

Future Tasks

Continue moving other components to libxrpl to allow us to move ledger and shamap.