Feature Request: support replacing fields with specified fake values

[Roger-luo]

Thanks for making the package! this makes it a lot easier for testing HTTP requests indeed. But I find that I'm not able to hide credentials that are not inside the header, my use case is the IBMQClient.jl, where I will need to use my user token to get an access token from the REST API, thus the response body will contain this credential, I wish to replace that
with a fake credential so that I can test it in CI, I currently have to do it manually since I didn't find any options to do so.

I tried to implement it myself, but due to my limited knowledge of the implementation, I didn't have any luck. it would be nice if this package could support this.

[christopher-dG]

I think we could do this in a nice way with a post-response recording hook, basically letting you pass in a mutating function where you could modify the response to your liking before it gets persisted to disk in recording mode.

Actually, that combined with a pre-request playback hook (modifying request parameters before comparing to the saved request) could replace the current ignore_query and ignore_headers for a more unified API.

For your use cases the hook might look like this:

function transform_before_save(r)
    body = JSON.parse(String(r.body))
    body["access_token"] = "<redacted>"
    r.body = Vector{UInt8}(json(body))
end

I'll have to do some more thinking about how to make this all work nicely, but it's definitely a use case that we should support.