add openenv-core pypi project and build instructions

[pankit-eng]

This change adds pypi project for openenv-core and adds a build script for the same as gh actions. Sample code to import:

from openenv_core.env_server import Environment, HTTPEnvServer, create_app
from dataclasses import dataclass

@dataclass
class MyAction:
    text: str

@dataclass
class MyObservation:
    response: str
    reward: float = 0.0
    done: bool = False

class MyEnvironment(Environment):
    def reset(self) -> MyObservation:
        return MyObservation(response="Ready")

    def step(self, action: MyAction) -> MyObservation:
        return MyObservation(
            response=f"Echo: {action.text}",
            reward=1.0,
            done=False
        )

# Create FastAPI app
env = MyEnvironment()
app = create_app(env, MyAction, MyObservation)

# Run with: uvicorn module:app --host 0.0.0.0 --port 8000