Allow client to be configured during initialization

[skewty]

This change allows for cleaner usage of Client class in context managers.

from nats.aio.client import Client, ClientConfig
from nats.micro.service import Service, ServiceConfig

async def main():
    config = ServiceConfig(name="demo-service", version="1.0.0", description="Demo service")
    async with Client(servers=["demo.nats.io"]) as nc, Service(nc, config) as service:
        group = service.add_group(name="demo")
        await group.add_endpoint(name="echo", handler=echo, subject="ECHO")