expression: change date add function return type v2

[mengxin9014]

What problem does this PR solve?

Issue Number: close #27573

Problem Summary:
fix the bug that a string like "20129" will not be judged as date type
change date_add and date_sub string_(int/string/real/decimal) function return type to string

What is changed and how it works?

What's Changed:
time.go
expression/builtin_time.go
expression/builtin_time_test.go
expression/builtin_time_vec_generated.go
types/time_test.go
cmd/explaintest/r/tpch.result

How it Works:
if the length of a string with no separator is 5, it will be date type
if use date_add or date_sub string_(int/string/real/decimal) function will return string type

Check List

Tests

• Unit test

Release note

change date_add and date_sub string_(int/string/real/decimal) function return type to string.

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[ti-chi-bot]

@mengxin9014: PR needs rebase.

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