Ensure backwards compatibility with ODFE

[dblock]

Coming from opensearch-plugins#12, enable upgrade to OpenSearch, while supporting backward compatibility.

• Rename Namespaces (e.g. com.amazon.opendistroforelasticsearch to org.opensearch)
• Rename Classes, Methods, Variables
• Rename Remaining Identifiers (e.g. Opendistro to OpenSearch).
• Rename, and Backwards Compatibility for Rest APIs
• Rename, and Backwards Compatibility for Settings
• Rename, and Backwards Compatibility for Indices
• Run in a backwards compatible way on top of OpenSearch 1.0 that has joined an ES 7.10.x cluster
• Run in a backwards compatible way on top of OpenSearch 1.0 that has joined an ODFE 1.13.x cluster
• Drop in replacement for the Opendistro version 1.13 of the plugin

[dbbaughe]

Since we've released I'm assuming this is done.