Previously, a "data" auth. resource knew how to check it's own existence by
accessing a global variable.
This patch accomplishes two things: it adds existence checking to all
kinds of resources, and moves these checks outside of `auth::resource`
itself and into `auth::service` (so that global variables are no longer
accessed).
This has the dual benefit of not enforcing copying on implementations of
the abstract interface and also limiting unnecessary copies.
As usual with Seastar, we follow the convention that a reference
parameter to a function is assumed valid for the duration of the
`future` that is returned. `do_with` helps here.
By adding some constants for root resources, we can avoid using
`seastar::do_with` at some call-sites involving `resource` instances.
The motivation behind this change is the idea that constructing a new
instance of an object is the job of the constructor.
One big benefit of this structure (with the addition of helpers for
convenience) is that calls for emplacing instances (like
`std::make_shared`, or `std::vector::emplace_back`) work without any
difficulty. This would not be true for static construction functions.
Different kinds of resources support different permissions. For example,
a keyspace supports the CREATE permission, which allows a user to
create tables in that keyspace. However, a table does not have an
applicable CREATE permission.
If a non-applicable permission is requested, an
`invalid_request_exception` is thrown.
