[keycloak-dev] Support for arbitrary byte arrays as HOTP keys

The current KeyCloak HOTP implementation assumes that a HOTP key (aka seed, aka initialization vector) is stored as string, and thus contains only printable characters. However, the HOTP standard (RFC 4226) doesn't impose any restrictions on key material; any arbitrary byte array is acceptable. Moreover, many hardware HOTP tokens are pre-programmed at the factory, and do contain non-printable seeds. Even though KeyCloak doesn't support hardware tokens out of the box, developers could implement it by extending KeyCloak and employing existing algorithms. Unfortunately, the existing convention (to store HOTP seeds as printable strings) makes this impossible. For the "password" credential type, the "value" field is already stored as Base64. I think "hotp" credentials could also be stored as Base64 or hex; another option would be to store the "value" field as BLOB (like it's already done for the "salt" field). I think I could produce a PR for this, I only need to know which scenario is preferred. Cheers, Mitya