feat: Add function calling from `{% completion %}`

[masci]

Working example:

import json

from banks import Prompt

def get_current_weather(location: str):
    """Get the current weather in a given location"""
    if "tokyo" in location.lower():
        return json.dumps({"location": "Tokyo", "temperature": "10", "unit": "celsius"})
    elif "san francisco" in location.lower():
        return json.dumps({"location": "San Francisco", "temperature": "72", "unit": "fahrenheit"})
    elif "paris" in location.lower():
        return json.dumps({"location": "Paris", "temperature": "22", "unit": "celsius"})
    else:
        return json.dumps({"location": location, "temperature": "unknown"})

p = Prompt("""
{% set response %}
{% completion model="gpt-3.5-turbo-0125" %}
    {% chat role="user" %}
           What's the weather like in {{ query }}?
    {% endchat %}
    {{ get_current_weather | tool }}
{% endcompletion %}
{% endset %}

{# the variable 'response' contains the result #}

{{ response }}
""")

print(p.text({"query": "Paris", "get_current_weather": get_current_weather}))

[coveralls]

coverage: 95.748% (+1.2%) from 94.541% when pulling 2cdbace275eb682c2031dae2773c63d1165b1827 on massi/function-calling into 05194bb309577884933f36f81532818edcae2dbf on main.