Support BatchLogProcessor without async runtime.

[lalitb]

More focused issue for logs (picked from #1437).

While the ideal approach would be to implement a custom asynchronous runtime (see #1506), which is currently a work in progress, an alternative solution could be to use a dedicated thread for uploading logs after reading from the queue. This method is commonly used in other language implementations.

C++: https://github.com/open-telemetry/opentelemetry-cpp/blob/d8ae09e53d6988a063364c86a8362053432a3f9f/sdk/src/logs/batch_log_record_processor.cc#L66

.Net: https://github.com/open-telemetry/opentelemetry-dotnet/blob/main/src/OpenTelemetry/BatchExportProcessor.cs

It seems .Net also has a concept of optionally using a shared pool for LogRecord, will create a separate issue to track this optimization.