* generalize `Keypair` to using `EncodableKey` trait in clap-v3-utils
* add associated `Pubkey` type to `EncodableKey`
* remove associated type `Pubkey` from `EncodableKey`
* rename `EncodableKey` associated function names
* remove default overrides for `{read,write}_file`
* resolve dependencies for test
* remove `pubkey_string` from `EncodableKey` trait