[Feature] datetime(3)默认值支持now(3)

[zhbdesign]

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[zy-kkk]

It's support in 2.0.0 #21487

[zhbdesign]

It's support in 2.0.0 #21487

2.0.0release无法用now(3)作为默认值建表

[zhbdesign]

It's support in 2.0.0 #21487

current_timestamp 已经支持了，能支持now(3)就更好了，而且2.0.0和2.0.1在flink写入的时候datetime类型有bug，doris已经在修复了

[zy-kkk]

It's support in 2.0.0 #21487

current_timestamp 已经支持了，能支持now(3)就更好了，而且2.0.0和2.0.1在flink写入的时候datetime类型有bug，doris已经在修复了

Flink CDC writing problem This PR has been fixed: #23092
Secondly, it is indeed more concise to use now as the default value function, we will consider