Add AggTrades (AGGTRADES) variant to WhatToShow enums

[bebop23]

Add AggTrades (AGGTRADES) variant to WhatToShow enums

Summary

Adds an AggTrades variant to both market_data::historical::WhatToShow and
market_data::realtime::WhatToShow, serialized on the wire as AGGTRADES.

IB requires AGGTRADES (not TRADES) for crypto contracts: TWS rejects
whatToShow=TRADES for crypto with error 10299 on both reqHistoricalData
and reqRealTimeBars. There was previously no way to express this through the
typed WhatToShow enums (and no raw-string escape hatch), so crypto bar
requests could not be made through the crate.

Changes

• historical::WhatToShow: new AggTrades variant + Display ("AGGTRADES")
  • FromStr mapping.
• realtime::WhatToShow: new AggTrades variant + Display ("AGGTRADES").
• Extended the existing WhatToShow unit tests (to_string / from_string
  round-trips) to cover AGGTRADES.

Purely additive — no existing variant, wire value, or behavior changes.

Verification

Cross-checked against a direct TWS-API reqRealTimeBars call with
secType=CRYPTO, exchange=ZEROHASH, whatToShow=AGGTRADES on an IB paper gateway:
bars flow (including zero-volume bars), whereas TRADES returns error 10299.
cargo test --lib what_to_show passes (historical to_string/from_string +
realtime display, all including the new AGGTRADES cases).

Downstream context

This unblocks crypto market data in the nautilus_trader Interactive Brokers
adapter, which currently has no way to request AGGTRADES through the typed
enums: nautechsystems/nautilus_trader#4389. That adapter pins ibapi 3.0.1, so
this change applies cleanly to the v3.0.1 line as well as main (the two
edited files are identical between them).