Create extensions directory and blank extensions.yml file in the same way as opensearch.yml

[ryanbogan]

Currently, users are required to manually create an extensions directory and an extensions.yml file before running extensions. It would be beneficial if the directory and an empty file were automatically generated in the same manner that opensearch.yml is created before the tar is generated.

[dbwiddis]

I think a better solution would be to allow this to be generated after startup with a REST API call. See opensearch-project/opensearch-sdk-java#356

[dblock]

@dbwiddis In some deployment scenarios writing a config file at runtime may be undesirable. Keep that in mind, we may need both.