Add transfer_expiry example

[vicsn]

Shows an example of simple transfer logic with a height- and nonce-based expiry.

Instead of only taking in a receiver and an amount, this new function also takes a block height as input.

[vicsn]

Compilation and proving succeeds, but the test fails with:

0 / 1 tests passed.
FAILED: test_transfer_public_expiry.aleo/test_expiry | rejected -- {
  program_id: test_transfer_public_expiry.aleo,
  function_name: test_expiry,
  arguments: [
    {
      program_id: transfer_public_expiry.aleo,
      function_name: transfer_public_with_expiry,
      arguments: [
        aleo1qr29hmz7qcrs90hxnjqq8akqlsn6t2nlrtfzlj3jy6lhvpxgyq9sgj9ger,
        1u64,
        1000000u32,
        {
          program_id: credits.aleo,
          function_name: transfer_public,
          arguments: [
            aleo1gzxe6lzcjh9fl92aeaxkayhnscdt4875w28eec82ykpcthnwygxspaqpwq,
            aleo1kanvv2cs88dpguwz5yrv890zz89rmycyk7qe6q4qm7vnz30flyzs56zae8,
            1000u64
          ]
        }
      
      ]
    }
  
  ]
}

We should revisit improving the error reporting after ProvableHQ/snarkOS#3934