feat(silentpayments): add experimental silent-payments sending support

feat(silentpayments): add experimental silent-payments sending support

- Adds CreateSpTx command to create transactions with silent payment
  outputs: this command creates signed transactions directly rather than
  PSBTs due to current limitations in secure shared derivation.

  It supports mixed recipients: regular addresses + silent payments.
  It DOES NOT support RBF for the created transactions.
  It generates signed transactions ready for broadcasting.

- Adds SilentPaymentCode command to create silent payment codes from
  public keys and network: the silent payment code generated is
  independent from any of the other stateful features of bdk-cli.

  This command is mainly intended for experimental use, do not lock any
  funds to the generated code if you don't know what you are doing and
  don't have the keys matching the public keys used.

- Adds bdk_sp dependency with "silent-payments" feature flag.
- Adds silent payment recipient parsing utility.
- Add README section for new silent payment commands.

Note: This is experimental functionality for testing only, not
recommended for mainnet use.

Signed-off-by: nymius <155548262+nymius@users.noreply.github.com>