feat: RequestOptions for individual requests

[64bit]

Option to allow per request configurable headers and query parameters.

This enables chaining of methods to optionally provide custom query or headers, for example (only for demonstration):

client
  .chat()
  .query(&[("limit", 10)])?
  .list()
  .await?

or

client
  .chat()
  .header("key", "value")?
  .query(&[("limit", 10)])?
  .list()
  .await?

or

client
  .chat()
  .headers(HeaderMap::new())
  .list()
  .await?