Implement --generate command

Includes logic to help users manage their keys:

- If the key is using a default PIN, we require the user to change it.
- We set the PUK equal to the PIN so the user doesn't need to remember
  them separately.
- We migrate the default management key to a new PIN-protected key.

Cargo.toml

@@ -14,13 +14,18 @@ edition = "2018"
 age-core = "0.5"
 age-plugin = "0.0"
 bech32 = "0.8"
+chrono = "0.4"
 console = "0.14"
+dialoguer = "0.8"
 elliptic-curve = "0.8"
 env_logger = "0.8"
 gumdrop = "0.8"
+hex = "0.4"
 log = "0.4"
 p256 = "0.7"
+rand = "0.8"
 sha2 = "0.9"
+x509 = "0.2"
 x509-parser = "0.9"
 yubikey-piv = { version = "0.3", features = ["untested"] }