[sdk-logs] add `forceFlush` operation to `LogRecordExporter`

[pichlermarc]

Description

Currently, the spec requires that the forceFlush operation may implemented by LogRecordExporter (see specification). However, we currently do not implement or call it.

This issue is considered done when:

• forceFlush was added to LogRecordExporter
• forceFlush is called from built-in LogRecordProcessors

Refs:

• opentelemetry-js/experimental/packages/sdk-logs/src/export/LogRecordExporter.ts

  Lines 21 to 33 in 903739c

  	export interface LogRecordExporter {
  	/**
  	* Called to export {@link ReadableLogRecord}s.
  	* @param logs the list of sampled LogRecords to be exported.
  	*/
  	export(
  	logs: ReadableLogRecord[],
  	resultCallback: (result: ExportResult) => void
  	): void;
  	/** Stops the exporter. */
  	shutdown(): Promise<void>;
  	}

• https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/logs/sdk.md#forceflush-2

[pichlermarc]

assigned this to myself since I'm working on this over at #6340 and I have a working diff + tests already.