[FEATURE]: Add PositionAveragePrice to ITradeUpdate

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your feature request related to a problem? Please describe.

ITradeUpdate currently returns
1) Total Position Size in Portfolio (PositionQuantity and PositionIntegerQuantity)
2) Trade Size (TradeQuantity and TradeIntegerQuantity)
3) Trade Price (Price)
https://olegra.github.io/Alpaca.Markets/api/Alpaca.Markets.ITradeUpdate.html

Seeing as it already returns the Total Position Size, it would be helpful to also return the Average Price of the Total Position Size.

### Describe the solution you'd like

ITradeUpdate currently returns
1) Total Position Size in Portfolio (PositionQuantity and PositionIntegerQuantity)
2) Trade Size (TradeQuantity and TradeIntegerQuantity)
3) Trade Price (Price)

Seeing as it already returns the Total Position Size, it would be helpful to also return the Average Price of the Total Position Size. It would save manual calculation, an extra API call, and ensure both local and Alpaca server values are synced.

Also maybe consider changing variable Price to TradePrice for better clarity?

Thanks!

### Describe alternatives you've considered

_No response_

### Anything else?

_No response_