move pynumaflow and its related entities to a new a sub-dir

[vigith]

➜  numaflow-python git:(move-under-a-dir)$ tree -L 2 .
├── CHANGELOG.md
├── CODE_OF_CONDUCT.md
├── CONTRIBUTING.md
├── development.md
├── docs
│   └── DOCKER_OPTIMIZATION.md
├── hack
│   ├── changelog.sh
│   └── update_examples.sh
├── LICENSE
├── Makefile
├── pynumaflow
│   ├── examples
│   ├── poetry.lock
│   ├── pynumaflow
│   ├── pyproject.toml
│   ├── pytest.ini
│   ├── README.md
│   ├── releases.md
│   └── tests
└── pynumaflow-lite
│   .....

[vigith]

Closed by #258