executor, infoschema: fix display of on update CURRENT_TIMESTAMP with decimal

[tangenta]

What problem does this PR solve?

Fix #11349.

What is changed and how it works?

1. refactor IsCurrentTimestampExpr into IsValidCurrentTimestampExpr. Here "valid" means the fractional seconds precision in column type equals to the one in on update clause.
2. print fractional seconds precision in on update clause when show create table and describe if necessary. For example, here is a table with the following schema:

create table t (
  a datetime not null default current_timestamp on update current_timestamp,
  b datetime(3) not null default current_timestamp(3) on update current_timestamp(3)
);

show create table t:

+-------+----------------------------------------------------------------------------------------+
| Table | Create Table                                                                           |
+-------+----------------------------------------------------------------------------------------+
| t     | CREATE TABLE `t` (                                                                     |
|       |   `a` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,         |
|       |   `b` datetime(3) NOT NULL DEFAULT CURRENT_TIMESTAMP(3) ON UPDATE CURRENT_TIMESTAMP(3) |
|       | ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin                            |
+-------+----------------------------------------------------------------------------------------+

desc t:

+-------+-------------+------+-----+----------------------+--------------------------------------------------+
| Field | Type        | Null | Key | Default              | Extra                                            |
+-------+-------------+------+-----+----------------------+--------------------------------------------------+
| a     | datetime    | NO   |     | CURRENT_TIMESTAMP    | DEFAULT_GENERATED on update CURRENT_TIMESTAMP    |
| b     | datetime(3) | NO   |     | CURRENT_TIMESTAMP(3) | DEFAULT_GENERATED on update CURRENT_TIMESTAMP(3) |
+-------+-------------+------+-----+----------------------+--------------------------------------------------+

Check List

Tests

• Unit test
• Integration test

Code changes

• Has exported function/method change

Related changes

• Need to cherry-pick to the release branch

[codecov]

Codecov Report

Merging #11480 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master    #11480   +/-   ##
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  Coverage   81.486%   81.486%           
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  Files          426       426           
  Lines        93103     93103           
=========================================
  Hits         75866     75866           
  Misses       11853     11853           
  Partials      5384      5384

[AilinKid]

/LGTM

[winkyao]

LGTM

[bb7133]

/run-all-tests

[bb7133]

LGTM

[bb7133]

/run-unit-test

[sre-bot]

cherry pick to release-3.0 in PR #11591

[sre-bot]

cherry pick to release-2.1 failed