Make OpenAI client more configurable

[franciscodr]

This pull request is the first iteration to improve how Xef instantiates the OpenAI client. The proposed changes are:

• Remove the default values for the properties of the Config data class in favor of providing a Default instance with such values.
• Implement a ConfigBuilder to allow the users to change the default behavior by assigning different values to the properties. For instance:

Config {
  organization = "new-organization"
}

• Add specific models to configure the timeout and retry policy for the HTTP client more intuitively. For example:

Config {
  httpClientRetryPolicy = HttpClientRetryPolicy.Incremental(
          interval = 250.milliseconds,
          maxDelay = 5.seconds,
          maxRetries = 5
        )
}