feat(otlp): add custom headers support for OTLP exporter

[CalvinAllen]

Summary

• Added OtlpHeaders dictionary property to TelemetryConfiguration for custom OTLP headers
• Created shared ConfigureOtlpExporter() method to reduce code duplication
• Headers are applied to all OTLP exporters (traces, metrics, logs)
• Updated README with documentation and Honeycomb example

Usage

var config = new TelemetryConfiguration
{
    ServiceName = "MyExtension",
    OtlpEndpoint = "https://api.honeycomb.io:443",
    UseOtlpHttp = true
};

config.OtlpHeaders["x-honeycomb-team"] = "your-api-key";
config.OtlpHeaders["x-honeycomb-dataset"] = "your-dataset";

VsixTelemetry.Initialize(config);

Test Plan

• Build succeeds
• Headers are correctly formatted as key1=value1,key2=value2 for OTLP exporter