OpenPGP Smart Card: getSupportedAlgorithms(), changePIN(), generateKey(), external_secrets_01 - #2565

This PR implements milestone 5 and adds a number of changes:

• OpenPGP Smart Cards now expose supported algorithms via OpenPGPSmartCard.getSupportedAlgorithms(), which is implemented by the yubikey and simulator backends.
• There is now an API to change the admin / user PIN of cards via OpenPGPSmartCard.changeUserPIN(oldCardPinProvider, newCardPinProvider), OpenPGPSmartCard.changeAdminPIN(oldCardPinProvider, newCardPinProvider).
• It is now possible to instruct the card to generate a key on-device, returning the public key component. This is done in preparation for milestone 7 and 8, which will allow high-level on-device key generation.
• The OpenPGP External Secret Keys document has been adopted by the OpenPGP working group and is now available here. This PR adopts the changes from revision 01, which flips the logic for empty locator hints to mean "unavailable" rather than "best effort".