add an api-version header to all requests

[ahl]

Closes #564.

[davepacheco]

I think this won't cover the new_with_client() case, right?

[ahl]

Oof. Good point! And 1. new_with_client() is actually the case that matter and 2. we can't modify a reqwest::Client nor can we convert it into a builder.

[ahl]

@davepacheco I think I need to change this progenitor interface more significantly--once we have a reqwest::Client e.g. from new_with_client there isn't anything we can do to glue in new our header. Some options:

// 1
Client::new_with_builder(reqwest::Client::builder().custom_settings());
// 2
Client::new_map(|builder| builder.customsettings());

Are there others? I think I prefer 2 in that it (I think) makes it clear to the user that the Client is in charge and is allowing you to make changes.

[davepacheco]

Yeah I was afraid of that.

Elsewhere I've had stuff like this accept a ClientBuilder (so, your option #1). Here's an example (this doesn't use the progenitor client, it's just an example of something that needs a reqwest client but wants to customize it). I think this communicates "you can provide your settings, and I will modify further[, and mine take precedence]". It looks nice but it's not as crisp as it looks because it's not clear which settings the function will customize and even if it were, it wouldn't necessarily be stable. I don't think there's a way around that, though.

In your option 2, it's possible to allow either one's settings to override the other's. I'm not sure if that's better or worse.

I don't have a strong feeling but I think (1) would be more composable? Looking at where my example is used, there's:
https://github.com/oxidecomputer/omicron/blob/5bf5f09545b7b74f809c099db5c0b315ea06f9be/end-to-end-tests/src/helpers/ctx.rs#L244-L255

Another example is in the Omicron TLS certificate tests, where I have this helper that keeps track of a few things needed to create a working client:
https://github.com/oxidecomputer/omicron/blob/5bf5f09545b7b74f809c099db5c0b315ea06f9be/nexus/tests/integration_tests/certificates.rs#L640-L649
https://github.com/oxidecomputer/omicron/blob/5bf5f09545b7b74f809c099db5c0b315ea06f9be/nexus/tests/integration_tests/certificates.rs#L683-L690

I think you could do all of this with your (2) but it would mean passing closures around through a few layers instead of ReqwestBuilders, so maybe it makes no difference, but seems a little more annoying to me.