Rewrite methods relate to the api

[RexWzh]

Hi there!
I've made some backward-compatible changes to several functions that I think would be more Julian. Additionally, I've added a new feature for api_key initialization. Please let me know if you have any issues or concerns about these changes.

[RexWzh]

This error might be caused by failed access to secrets.OPENAI_API_KEY. The tests are passing successfully here.

[RexWzh]

Add new funciton get_usage_status:

provider = OpenAI.OpenAIProvider(ENV["OPENAI_API_KEY"], "https://api.openai.com/v1", "")
(; quota, usage, daily_costs) = get_usage_status(provider, numofdays=5)
println("Total quota: $quota")
println("Total usage: $usage")
costs = [sum(item["cost"] for item in day.line_items) for day in daily_costs]
println("Recent costs(5 days): $costs")

Output:

Total quota: 5.00004
Total usage: 0.0003299
Recent costs(5 days): Real[0.03299, 0, 0, 0, 0]

It is a wrapper of

curl -H "Content-Type: application/json" \
     -H "Authorization: Bearer $api_key" \
     -X GET \
     https://api.openai.com/v1/dashboard/billing/usage\?start_date\=$start_date\&end_date\=$end_date

curl -H "Content-Type: application/json" \
     -H "Authorization: Bearer $api_key" \
     -X GET \
     https://api.openai.com/v1/dashboard/billing/subscription

[roryl23]

This looks good to me, thank you for the PR!