chore: update for Preview 20

_work in progress: needs CLI v20.0.3, which must actually include latest bindings logic. Somehow 20.0.2 does not, even though it was tagged post-multi-auth merge https://github.com/stellar/soroban-tools/commits/main/)._

- Pin soroban-cli to latest version, 20.0.3
- Update the contract's SDK version
  - **Copy in `abundance` source from `token` contract** at stellar/soroban-examples#277. Note that I **did not author most of the changes in the `contracts/abundance` folder**. I _only_ copied in the changes originally authored in the `soroban-examples` repo, then updated for the single tweak that this `abundance` contract already had: 1. comments on the `mint` method; 2. slightly modified `mint` behavior.
- New CLI's typescript-bindings-generated libraries now return an `AssembledTransaction`, which has a `result` getter and a `signAndSend` method, as explained in stellar/stellar-cli#1034
- Use `stellar-sdk` rather than `soroban-client`

.cargo/config.toml

@@ -1,7 +1,7 @@
 # paths = ["/path/to/override"] # path dependency overrides
 
 [alias] # command aliases
-install_soroban = "install --git https://github.com/AhaLabs/soroban-tools --rev c7fb7e08ba8efa9828d9df863d991558f269e35b --root ./target soroban-cli --debug"
+install_soroban = "install --version 20.0.2 --root ./target soroban-cli --debug"
 b = "build --target wasm32-unknown-unknown --release"
 # c = "check"
 # t = "test"