Feat: Add support for setting struct field size in tracelogging_dynamic for Rust

Description

When working with ETW events, I noticed we are missing the functionality to create a struct, and then populate it, and lastly, set it struct size. This workflow is typical in tracelogging_dynamic scenarios, where not all the data for the struct is known ahead of time. This facilitates a lot of workflows and code structure.

Example:

        use tracelogging_dynamic as tld;
        
        let mut field_count  = 2;
        
        let event = tld::EventBuilder::new();
        
        // Create struct
        event.add_struct("Struct", field_count, 0);
        
        event.add_str8("string1", "value", tld::OutType::Default, 0);
        event.add_str8("string2", "value", tld::OutType::Default, 0);
        
        if let Some(value) = Some("hello, world".to_string()) {
          event.add_str8("greeting",value, tld::OutType::Default, 0);
          field_count = field_count + 1;
        }
        
        event.set_struct_field_count("Struct", field_count); // <-- Missing method
    }

Other implementations

Other tracelogging implementation have this method:
https://github.com/open-telemetry/opentelemetry-dotnet-contrib/blob/3698be915322216aa126a67f19100b1cd93bb6c9/src/OpenTelemetry.Exporter.Geneva/External/TraceLoggingDynamic.cs#L1746

Feature Request:

Implement set_struct_field_count in EventBuilder struct for Rust in tracelogging_dynamic crate.

Start by locating the EventBuilder implementation in the tracelogging_dynamic crate and review add_struct alongside the linked TraceLoggingDynamic.cs implementation. Implement and validate set_struct_field_count so the example's optional-field workflow can update the struct count after fields are added.