feat(openai): add token usage and cost tracking callback

[mspronesti]

Hi @gventuri,
this PR aims at supporting token usage count and cost tracking for OpenAI llms, adapting and simplifying langchain's implementation.

Supposed usage:

"""Example of using PandasAI with a Pandas DataFrame"""

import pandas as pd
from data.sample_dataframe import dataframe

from pandasai import PandasAI
from pandasai.llm.openai import OpenAI
from pandasai.helpers.openai_info import get_openai_callback

df = pd.DataFrame(dataframe)

llm = OpenAI()

# conversational=False is supposed to display lower usage and cost
pandas_ai = PandasAI(llm, enable_cache=False, conversational=True)

with get_openai_callback() as cb:
    response = pandas_ai(df, "Calculate the sum of the gdp of north american countries")

    print(response)
    print(cb)
#  The sum of the GDP of North American countries is 19,294,482,071,552.
#  Tokens Used: 375
#	Prompt Tokens: 210
#	Completion Tokens: 165
# Total Cost (USD): $ 0.000750

Of course it also works with AzureOpenAI.
Missing tests for now.

• closes Add support for token usage count #318
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.

[mspronesti]

@gventuri Ready to go :)

[gventuri]

@mspronesti thanks a lot, just merged!