test: add BeaconDepositDataSpecTest spec coverage

[shane-moore]

Parent

Part of #954 (split from PR #840).

What

Add BeaconDepositDataSpecTest to the spec test suite. Mirrors Go's GenerateETHDepositData() which validates deposit signing root computation.

Reference implementation

Branch test/simple-spec-test-types, file: anchor/spec_tests/src/types/beacon_deposit_data.rs

Details

• Parses validator public key, withdrawal credentials, and fork version from fixture
• Constructs a DepositMessage with ChainSpec::mainnet().max_effective_balance
• Computes domain via spec.compute_domain(Domain::Deposit, fork_version, Hash256::ZERO)
• Validates signing_root matches expected value
• Fixture validator PK has no 0x prefix (test prepends it)
• 1 fixture: beacon.DepositDataSpecTest_deposit_data_root_and_ssz.json

Changes needed

1. New file: anchor/spec_tests/src/types/beacon_deposit_data.rs
2. Add module + re-export in anchor/spec_tests/src/types/mod.rs
3. Add dispatch arm "beacon.DepositDataSpecTest" in anchor/spec_tests/src/runner/types_dispatcher.rs