Fix/soroban auth signing with ledger

[elizabethengelman]

What

[TODO: Short statement about what is changing.]

Why

There is currently a workaround for this, in that users can build the tx and then pipe that into simulate/sign/send.

cargo run contract invoke --id hello-world --source ledger --sign-with-ledger --build-only -- inc | \
  cargo run tx simulate --source ledger |  \
  cargo run tx sign --sign-with-ledger | \
  cargo run tx send

This PR makes that workflow a bit more streamlined by allowing users to sign contract invoke txs with a ledger directly.

Testing

local source key, ledger as auth key

stellar contract invoke --source local-key-1 --id hello-world --network local  \
-- auth  --addr ledger --world hi

• worked before this change ❌

   thread 'main' panicked at cmd/soroban-cli/src/config/secret.rs:121:31:
   Ledger does not reveal secret key
   note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
  

• works after this change

---

ledger as source key, local auth key ✅

stellar contract invoke --source ledger --id hello-world --network local --sign-with-ledger \ 
 -- auth --addr local-key-1 --world world

• worked before this change
  • ❌ error: Ledger cannot reveal private keys
• works after this change ✅

ℹ️ Signing transaction: e78ea889a32f7d6095ac0a33b0c9287d3caa6150a369afe4d2edcd14b23fce3e
📅 CAIQMCKL4ZKLPXIDOKXG2YW54GLTDCBX7FQYWMV5R2FXVLS6TVSH52FA - Success - Event: [{"symbol":"auth_event"}] = {"map":[{"key":{"symbol":"world"},"val":{"symbol":"world"}}]}
"GBZ2322JZAO2AQF3XT2THWPDIVESPOAD4R5SYFW3EHOA6ATVLCCSUAXA"

---

ledger as source key, ledger as auth key

stellar contract invoke --source secure-key-1 --id hello-world --network local  \
-- auth  --addr secure-key-2 --world hi

• worked before this change
  • ❌ error: Secure Store does not reveal secret key
• works after this change

Known limitations

[TODO or N/A]