Support tidb_allow_auto_random_explicit_insert as a global variable

[kolbe]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mysql> set global allow_auto_random_explicit_insert=on;

ERROR 1105 (HY000): Variable 'allow_auto_random_explicit_insert' is a SESSION variable and can't be used with SET GLOBAL

2. What did you expect to see? (Required)

It should be possible to set allow_auto_random_explicit_insert as a global variable. The rationale for disallowing this in #17102 (comment) was that a global variable couldn't meet the need of some specific use case, but the rationale was flawed. A variable that can be set globally can also be set on a per-session basis.

3. What did you see instead (Required)

4. Affected version (Required)

v4.0.3

5. Root Cause Analysis

[glorv]

/label component/executor