Add changelog for December 11, 2024 (#106)

Co-authored-by: quantstruct[bot] <190039098+quantstruct[bot]@users.noreply.github.com>

fern/changelog/2024-12-11.mdx

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+1. **Use OpenAI Chat Completions in your Assistant**: you can now more easily integrate your Assistant with OpenAI's [chat completions sessions](https://platform.openai.com/docs/api-reference/chat) by specifying `messages` (an array of `OpenAIMessage` objects) and an `assistantId` (a string). Each `OpenAIMessage` in turn consists of a `content` (a string between 1 and 100,000,000 characters) and  a role (between *assistant*, *function*, *user*, *system*, *tool*). This makes it easier to manage chat sessions associated with a specific assistant. Refer to the `ChatDTO`, `OpenAIMessage` schemas  in [our API docs](https://api.vapi.ai/api) to learn more.
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+2. **Update Subscription Email on Billing Page**: you can now customize which email address appears on your Vapi invoices through the updated billing page > [under payment history](https://dashboard.vapi.ai/org/billing). You can specify an email address (in addition through physical address and tax id) - read more in [our docs](https://docs.vapi.ai/quickstart/billing#how-do-i-download-invoices-for-my-credit-purchases).