rpc: Implement z_importviewingkey for Sapling

[mnm458]

Summary

Implements z_importviewingkey RPC method for Sapling extended full viewing keys (issue #80).

• Imports a Sapling extfvk (zxviews/zxviewtestsapling) so the wallet can track transactions without holding spending authority. Accounts are stored with AccountPurpose::ViewOnly.

  • Errors if the wallet already holds the spending key for the given viewing key (matching zcashd behavior).
  • Adds fetch_account_birthday to json_rpc::utils as a shared helper (also needed by Implement z_importkey and z_exportkey for Sapling #400 — whichever merges first provides it).
  • Includes unit tests for parameter validation, key decoding, address derivation, encoding roundtrips, and rejection of invalid/wrong-network/spending keys.

  Why implement this despite zero usage in the survey? Watch-only wallets are a fundamental capability for exchanges, auditors, and operators who need to monitor balances without holding spending keys. As zcashd is deprecated, this becomes the only path for users to import existing viewing keys into the new stack. It also supports the separation-of-concerns model where spending keys stay on cold storage while
  a hot node tracks balances.

  Test plan

  • cargo build — clean
  • cargo test -p zallet import_viewing_key — 13/13 passing
  • Integration test with live wallet DB (blocked on test: Need integration tests for zcashd migration #405 / Write a test harness for checking equivalence of zcashd and Zallet output #16)

  Close rpc: Implement z_importviewingkey #80