Add ForceFlush to Tracing SDK

[kipwoker]

What are you trying to achieve?

I want to add ForceFlush to Tracing SDK like it was done for the Shutdown method.

Additional context.

I'm using OpenTelemetry for AWS lambda functions. And I'm using dotnet opentelemetry library to achieve that. After each call, lambda goes to sleep and freezes background threads. Therefore I need to send activities to the exporter forcibly if I want to use the batch processor instead of the simple one.

Thankfully, BaseProcessor already contains a ForceFlush method that works perfectly in this case.

There is a PR that I've made for the dotnet library: open-telemetry/opentelemetry-dotnet#1837
My PR is based on a similar Java PR, which you can find here. And the Java PR has already been merged.

cc: @reyang @utpilla @cijothomas

[Oberon00]

Seems it was more or less an oversight that this was not there from the beginning together with Shutdown #1073 (comment)