Following commit makes sure that `build` api can accept external
secret and allows currently `NOOP` `podman-remote build -t tag
--secret id=mysecret,src=/path/on/remote` to become functional.
Just like `docker` following api is a hidden field and only exposed to
`podman-remote` but could document it if it needs exposed on `swagger`.
Signed-off-by: Aditya Rajan <arajan@redhat.com>