Gateway: add on_start functions to stubs

Add on_start function & add env module for determining if we are runn…

…ing locally or remotely (#160)

- Adds an on_start lifecycle method to taskqueues/endpoints
- Adds an is_local method
- Adds a local entrypoint decorator which can be used like this:

```import time

from beta9 import function
from beta9.env import local_entrypoint

def test_func():
    return True # this value will be available in some_func

@task_queue(cpu=0.1, on_start=test_func)
def some_func(context):
    print(context.task_id)
    print(context.on_start_value)
    for i in range(1000):
        print(i)
        time.sleep(0.01)
    return "hi"


# This will be executed automatically when running locally (like if __name__ == '__main__')
@local_entrypoint
def main():
    print("hi there")
```

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Co-authored-by: Luke Lombardi <luke@beam.cloud>