[SPARK-29671][SQL] Simplify string representation of intervals

[MaxGekk]

What changes were proposed in this pull request?

In the PR, I propose to changed CalendarInterval.toString:

• to skip the week unit
• to convert milliseconds and microseconds as the fractional part of the seconds unit.

Why are the changes needed?

To improve readability.

Does this PR introduce any user-facing change?

Yes

How was this patch tested?

• By CalendarIntervalSuite and IntervalUtilsSuite
• literals.sql, datetime.sql and interval.sql

[MaxGekk]

ping @cloud-fan @dongjoon-hyun

[SparkQA]

Test build #113127 has finished for PR 26367 at commit 12d3583.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #113130 has finished for PR 26367 at commit 662eeef.

• This patch fails SparkR unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[MaxGekk]

jenkins, retest this, please

[SparkQA]

Test build #113157 has finished for PR 26367 at commit 662eeef.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[MaxGekk]

jenkins, retest this, please

[SparkQA]

Test build #113167 has finished for PR 26367 at commit 662eeef.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[dongjoon-hyun]

+1, LGTM. Thank you, @MaxGekk , @cloud-fan , @maropu .
This looks really nicer.

Merged to master.