Ability to override params, such as those for the XMLHttpRequest

[natoverse]

Some parameters (e.g., timeout) are not overridable but rather assumed to only come from the schemas. The ability to provider a deeper override would be really convenient. An easy way to accomplish this could be to allow the use of a "params" plugin that gets applied last in the chain before xhr, so it can override anything that has been auto-set or calculated.