Add plugin setting for new SQL module

[dai-chen]

Issue #, if available: #543

Description of changes: Add a plugin setting so user can choose to disable new SQL parser and engine if any issue found.

1. The setting is false (new query engine disabled) by default: This is temporary during this transition period. The new engine will be enabled by default in near future: https://github.com/dai-chen/sql/blob/add-new-sql-module-setting/docs/user/admin/settings.rst#opendistro-sql-engine-new-enabled
2. Add a note paragraph in new docs to remind user to enable new engine beforehand: https://github.com/dai-chen/sql/blob/add-new-sql-module-setting/docs/user/dql/expressions.rst
3. Enable new engine before comparison test and other IT in integ-test and doc in doctest (by a temporary doc).

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[penghuo]

LGTM

[chloe-zh]

LGTM, thanks for adding the setting!