sp-api: Use macro to detect if `frame-metadata` is enabled (#4117)

While `sp-api-proc-macro` isn't used directly and thus, it should have
the same features enabled as `sp-api`. However, I have seen issues
around `frame-metadata` not being enabled for `sp-api`, but for
`sp-api-proc-macro`. This can be prevented by using the
`frame_metadata_enabled` macro from `sp-api` that ensures we have the
same feature set between both crates.