feat: symmetric key notifications

[dan-da]

closes #161.

Implements public announcements encrypted via symmetric keys

This is most useful for utxos that transfer funds within a single wallet such as change addresses.

Compared to on-chain pub-key public announcements:

Data encrypted with symmetric keys is smaller than data encrypted with asymmetric keys so there is a blockchain space (and thus fee) savings.

Compared to off-chain expected utxos:

symmetric-key announcements exist on the blockchain and thus are immune to local data-loss situations. off-chain expected utxos do not use any blockchain space but require that the wallet holder make ongoing backups of wallet state and never lose them.

Design choices, please review:

1. SymmetricKey presently uses Aes256Gcm. Is this our final choice?

2. Symmetric keys are derived from a seed and are unique for each utxo. (rather than using a single sym-key per wallet shared for all utxo.) This is because we want the receiver_id to be unique to avoid linking utxos.

3. Symmetric keys derive from the same root seed WalletSecret::secret_seed as generation addresses but the derivation in ::nth_symmetric_key() uses a SYMMETRIC_KEY_FLAG. This means that end-user should only require a single mnemonic phrase for the root seed. A symmetric-key derivation counter must be stored in wallet state.

4. efficiency: SymmetricKey presently derives all fields from seed as needed, rather than pre-calc and store when created. see doc-comment for the struct.

5. change notify method now defaults to on-chain symmetric key, rather than offchain (expected utxo) in dashboard and neptune-cli. (note: this could easily be made an end-user option)

Summary of changes:

Cargo.toml:

• aead dep now requires feature std so that the aead::Error impls std::error::Error trait.

New code:

• add struct AnnouncedUtxo to represent a found/claimed utxo and its secrets.
• add enum UtxoNotifyMethodSpecifier that further simplifies create_transaction() params and impl.
• add struct SymmetricKey
• add WalletSecret::next_unused_symmetric_key() and ::nth_symmetric_key()
• add WalletState::get_known_symmetric_keys()

Modified code:

• added SymmetricKey to SpendingKeyType and ReceivingAddressType.
• added some missing doc-comments
• minor refactors to generation_address to match and share code with symmetric_key.
• update UtxoNotificationPool::scan_for_expected_utxos()
• update WalletState::scan_for_announced_utxos() to include symmetric keys.
• update WalletState::find_spending_key_for_utxo() to include symmetric keys.
• update WalletState::update_wallet_state_with_new_block()
• update GlobalState::create_transaction(). simplify args and impl.
• update rpc_server::send() and send_to_many() to accept change_utxo_notify_method param.
• dashboard and neptune-cli now pass OffChainSymmetricKey flag for send() parameter change_notify_method.

New Tests:

• symmetric_key::test::test_encrypt_decrypt()
• symmetric_key::test::scan_for_announced_utxos_test()

[dan-da]

closing in favor of #171.