apko `DiscoverKeys` has a panic on non-rsa jwks key that causes crash during key discovery

DiscoverKeys in pkg/apk/apk/implementation.go unconditionally type-asserts JWKS keys as rsa.PublicKey without checking the key type. If a repository JWKS endpoint returns a non-RSA key e.g. EC, the unchecked assertion panics and crashes apko. This affects any workflow that initializes the APK...