[FEATURE] Add new data type: `PERIOD`

Is your feature request related to a problem?

MySQL has no such type, at least visible to user, but having a such type we can simplify processing few datetime functions.

... number in YYMMDD, YYMMDDhhmmss, YYYYMMDD, or YYYYMMDDhhmmss format. If the argument includes a time part, it may optionally include a fractional seconds part.

See PERIOD_ADD/PERIOD_DIFF functions in MySQL docs for reference.
The proposed solution would help fixing NOW() + 0 issue, see discussion in #754 and https://github.com/Bit-Quill/opensearch-project-sql/pull/92.

What solution would you like?

A new data type PERIOD

1. Responsible for parsing and formatting
2. Couldn't be casted to numeric types
3. Expose a numeric type for user
4. Has operators + and - overloaded with integers.

This will also simplify UNIX_TIMESTAMP implementation (#835) https://github.com/opensearch-project/sql/blob/e04d6f886aa57ee4fdd8e632127bb5d8a339113f/core/src/main/java/org/opensearch/sql/expression/datetime/DateTimeFunction.java#L915-L938

Other functions affected: all listed in #754, EXTRACT, UTC_DATE, UTC_TIME and UTC_TIMESTAMP (link).

What alternatives have you considered?

Keep using numeric type.

Do you have any additional context?

#855
#722
#835

Start with core/src/main/java/org/opensearch/sql/expression/datetime/DateTimeFunction.java, especially the UNIX_TIMESTAMP implementation referenced in the issue. Read the linked MySQL PERIOD_ADD and PERIOD_DIFF documentation and review issues #754, #835, #855, and #722 for affected behavior. Done means a PERIOD type supports parsing and formatting, exposes a numeric value without numeric casts, and defines integer + and - operators.