Add test harness

[igalshilman]

pip install restate_sdk[harness]

greeter = Service("greeter")

@greeter.handler()
async def greet(ctx: Context, name: str) -> str:
    return f"Hello {name}!"

app = restate.app(services=[greeter])

import restate

with restate.test_harness(app) as harness:
    cli = harness.ingress_client()
    print(cli.post("/greeter/greet", json="Alice").json())

[slinkydeveloper]

I think this should be latest, otherwise we need to bump the python sdk every time we update the runtime. Plus i find counterintuitive that we decide here to pin to some version X.

Let the user override the restate image though, so everyone's happy and can pin to a specific restate version if they need to.

[igalshilman]

yah totally, it is a draft for a reason.