[SPARK-38535][SQL] Add the datetimeUnit enum and use it in TIMESTAMPADD/DIFF

[MaxGekk]

What changes were proposed in this pull request?

In the PR, I propose to add new enum datetimeUnit and re-use it in the datetime "function"s TIMESTAMPADD (see SPARK-38195) and TIMESTAMPDIFF (see SPARK-38284):

SqlBaseParser.g4:

datetimeUnit
    : YEAR | QUARTER | MONTH
    | WEEK | DAY | DAYOFYEAR
    | HOUR | MINUTE | SECOND | MILLISECOND | MICROSECOND
    ;

Why are the changes needed?

1. The enum will allow to document the list of supported units in the grammar which should improve user experience with Spark SQL.
2. Switching to unified parse error should improve UX too.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

By running test suites for TIMESTAMPADD/TIMESTAMPDIFF:

$ build/sbt "test:testOnly *SQLKeywordSuite"
$ build/sbt "sql/testOnly org.apache.spark.sql.SQLQueryTestSuite -- -z timestamp.sql"

[cloud-fan]

LGTM, please fix conflicts

[MaxGekk]

Merging to master. Thank you, @cloud-fan for review.

[melin]

datetimeUnit argument can also have a prefix of SQL_TSI_. For example, you could use either DAY or SQL_TSI_DAY
https://database.guide/timestampadd-examples-mysql/
@MaxGekk