…ion (#180)

Rework on `OpenSearchDataType`:
* Add data types for those classes are not defined in `ExprCoreType`.
* Address #180 (comment)
* Remove `TextKeywordValue`.
* Add changes according to the PR review. #180
* Update `IndexMapping::parseMapping` function.
* Add `OpenSearchDataType::resolve` function.
* Add new constructor for `OpenSearchTextType`.
* Make `fields` and `properties` in `OpenSearchDataType` readonly. Update tests and mapping parser.
* Move `getFields` from `OpenSearchDataType` to `OpenSearchTextType`. Update tests.
* Rewrite `traverseAndFlatten` according to #180 (comment)
* Minor comment fix.
* A fix to avoid breaking changes.
* `typeName` and `legacyTypeName` to return different type names.
* Change `typeof` function and corresponding tests.
* Move `convertTextToKeyword` from `ScriptUtils` to `OpenSearchTextType`. Update tests.
* Update UT for `typeof` function.
* Make all instances of `OpenSearchDataType` and of derived types singletones as much as possible.
* Make string representations of all `ExprType`s uppercase.
* Remove functions from `IndexMapping` used in tests only.

Signed-off-by: Yury-Fridlyand <yuryf@bitquilltech.com>
Signed-off-by: Yury-Fridlyand <yury.fridlyand@improving.com>

…have the common style.

Signed-off-by: Yury-Fridlyand <yury.fridlyand@improving.com>

…mapping-use

Signed-off-by: Yury-Fridlyand <yury.fridlyand@improving.com>

…t and keyword.

Signed-off-by: Yury-Fridlyand <yury.fridlyand@improving.com>

Signed-off-by: Yury-Fridlyand <yury.fridlyand@improving.com>

Signed-off-by: Yury-Fridlyand <yury.fridlyand@improving.com>

Signed-off-by: Yury-Fridlyand <yury.fridlyand@improving.com>