Extract the shared amount parameter into a reusable payments type

[hashbender]

Extracting logic from zcash#531. The idea is to have !531 as small as possible, and merge relatively unrelated changes to main, to decrease the reviewer time on the actual feature, while we're merging NU6.3 related changes.

---

Move the AmountParameter request type out of z_send_many into json_rpc::payments (with accessor methods) and extract the shared build_request amount-parsing helper, so the type and parsing can be reused by other send-path RPC methods. Update z_sendmany, the method trait, and the OpenRPC generation to reference the relocated type.

---

Mirror-of: zcash#535
zcash#535

[tenki-reviewer]

Review Complete
No issues found!

Risk: 🟢 Low (15/100) — no findings · 441 LOC across 9 files

---

No issues found that meet confidence and severity thresholds. One low-severity proptest coverage note was identified but fell below the confidence threshold for inclusion.

Files Reviewed (9 files)

Cargo.toml
supply-chain/config.toml
supply-chain/imports.lock
zallet/Cargo.toml
zallet/src/components/json_rpc/methods.rs
zallet/src/components/json_rpc/methods/openrpc.rs
zallet/src/components/json_rpc/methods/z_send_many.rs
zallet/src/components/json_rpc/payments.rs
zallet/src/components/json_rpc/utils.rs