Add Text and Keyword Data Type

[penghuo]

Description of changes

1. Add the Text and Keyword Data Type in Elasticsearch Storage Engine.
2. Redefine ExprValue interface.
   1.add the observer method to retrieve the value.
   2. add the compareTo method.
   3. add the equlas method.
3. Simplify the equal and notEqual operator definition in the BinaryPredicateOperator.
4. Add the doc for Data Types. https://github.com/penghuo/sql/blob/text-keyword-data-type/docs/user/general/datatype.rst

To Reviewers

1. I am planning an follow up PR to redefine all the operators in BinaryPredicateOperator. The formal doc for operator will been added accordingly.

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

[dai-chen]

LGTM. Thanks for this refactoring!