Refactor ExecutorServiceFactory out of GcsOptions

The SDK harness uses GcsOptions.ExecutorServiceFactory  [1] to process instruction requests [2]. I found this dependency really surprising, and it also happens to be unnecessary afaict, so ExecutorServiceFactory should probably get factored out into its own class.

[1] https://github.com/apache/beam/blob/39dab79b7bd56a47c0f2a02033ab4ca3bd4a67e2/sdks/java/extensions/google-cloud-platform-core/src/main/java/org/apache/beam/sdk/extensions/gcp/options/GcsOptions.java#L136-L158

[2] https://github.com/apache/beam/blob/39dab79b7bd56a47c0f2a02033ab4ca3bd4a67e2/sdks/java/harness/src/main/java/org/apache/beam/fn/harness/FnHarness.java#L170