feature: add public order properties

[Romazes]

Description

Adds PublicOrderProperties so an algorithm can opt into Public.com extended-hours trading and choose margin or cash-only buying power per order. The Lean.Brokerages.Public plugin reads these and maps OutsideRegularTradingHours to the request's equityMarketSession field and UseMargin to its useMargin field.

DefaultOrderProperties = new PublicOrderProperties
{
    OutsideRegularTradingHours = true, // route equity orders to the EXTENDED session
    UseMargin = false                  // use cash-only buying power
};

Related PR(s)

N/A

Related Issue

N/A

Motivation and Context

Public.com supports an extended trading session and a per-order choice between margin and cash-only buying power. Lean had no order-properties type to express either, so these options could not be set from an algorithm.

Requires Documentation Change

No.

How Has This Been Tested?

Manually placed equity orders through the Lean.Brokerages.Public plugin and confirmed the place-order request carried equityMarketSession (CORE/EXTENDED) and useMargin as set on the properties.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Refactor (non-breaking change which improves implementation)
• Performance (non-breaking change which improves performance)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Non-functional change (xml comments/documentation/etc)

Checklist:

• My code follows the code style of this project.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• My branch follows the naming convention bug-<issue#>-<description> or feature-<issue#>-<description>