Provide methods for closing and create the httpx session in AsyncFirecrest

[ekouts]

One of the issues mentioned in https://github.com/eth-cscs/pyfirecrest/issues/94 .

The code that was failing could look like that:

...
async def get_systems():
    await _client.create_new_session()
    systems = await _client.all_services()
    await _client.close_session()

asyncio.run(get_systems())
asyncio.run(get_systems())