Draft: Make response extensions from reqwest::Response available in octorust::Response

[ctsrc]

This PR makes http::Extensions from reqwest::Response available in octorust::Response.

This is a minimal draft that made it useable for me for my use-case.

Submitting this PR in draft state so that feedback can be provided on whether this is of interest to you guys if I add a bit more polish.

See also #97 that describes the original motivation for this PR.

[ctsrc]

cc @augustuswm Would you guys be open to merge this sort of addition? And is directly editing github/src/lib.rs the right way to do it, like I'm doing in this draft? Or is there a tool or template that generates it and I should make the edits elsewhere instead, so that the generated result becomes like this?

[augustuswm]

So to implement something like this it would be a change to source in https://github.com/oxidecomputer/third-party-api-clients/tree/main/generator/src . This is binary that generates all of the clients. In particular I think https://github.com/oxidecomputer/third-party-api-clients/blob/main/generator/src/main.rs#L2380 is likely the starting point to look at. You can then use make github to regenerate the client. It may contain formatting / clippy issues, but those are fine to leave as I'll address them in an integration PR.