[lwe] add dimension / size to the key attribute

Right now the key attribute doesn't store the number of polynomials that the secret key consists of. Even though it is most typically n = 1, it can in theory be any natural number. We have a KeyAttr attached to the secret/public LWE keys, and we can extend this attr to hold that information.

Note that there is also a size / dimension integer on the ciphertext space, and a ciphertext has a ciphertext space attr and a key attr - we will want a verification on the type that the key dimension and ciphertext dimension match I think.