Stores a custom API key from an LLM provider in your account. This can be done in
one of two ways:
As part of a custom endpoint. If you define a custom endpoint, you can reference
a custom API key. This will be sent to the endpoint as part of the request.
To use your own API keys with the standard providers. If any of your custom API
keys match a provider name and you pass use_custom_keys=True to the
/chat/completions endpoint, then this API key will be used, using your own
account with the provider directly.