Request: support arbitrary subresources

[MikailBag]

This PR only changes low-level Request; Api is left untouched.

This way users can work with exotic subresources (such as certificatesigningrequests/approval) without directly manipulating URLs.

[clux]

Looks reasonable to me. You're only breaking Request and not Api.

Do you think we should maybe add generic helpers for it ala Api::get_subresource::<U> in Api with an extra generic parameter for the output type?

[clux]

Just to clarify, I am completely happy with this, and thank you!

But I do think we should have the same support in Api:

let api: Api<Pod> = Api::namespaced(..)
api::patch_subresource::<Scale>("scale", "mypod", PatchParams, Patch);
api::get_subresource::<Scale>("scale", "mypod");
api::replace_subresource::<Scale>("scale", "mypod", PostParams, data);

even though it looks like the generic types on these 3 methods would overlap in the CSR case as the approval response looks like it's a CSR again:

let api: Api<CertificateSigningRequest> = Api::all(..)
api::patch_subresource::<CertificateSigningRequest>("approval", "mycsr", PatchParams, Patch);
api::get_subresource::<CertificateSigningRequest>("approval", "mycsr");
api::replace_subresource::<CertificateSigningRequest>("approval", "mycsr", PostParams, data);

[clux]

That said, the double string parameter is a bit ugly. Maybe we should leave it in Request for now.

As a follow-up, happy with PRs doing the above, or creating a cheap subresource shortcut (ala scale) for CSRs explicitly in Api.

[clux]

Released in 0.53.0. Thanks again.