ddl: fix problem with modifying bit column

[tangenta]

What problem does this PR solve?

Unexpected error:

create table t (a bit(1));
alter table t modify column a bit(20);
(1105, 'unsupported modify column unsigned false not match origin true')

Similarly,

create table t (a year);
alter table t modify column a year;
(1105, 'unsupported modify column unsigned false not match origin true')

When creating a bit column, the flag in the fieldType is set to Unsigned. However, when modifying it, the Unsigned flag of the new column is forgotten to be set, leading to the error above.

What is changed and how it works?

• Extract the logic of setting flags as setFlagByFieldType.
• Add setFlagByFiledType() to getModifiableColumnJob.

Check List

Tests

• Unit test
• Integration test
• Manual test

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch

Release note

• Fix problem with modifying bit column

[tangenta]

/run-all-tests

[winkyao]

LGTM

[AilinKid]

LGTM

[tangenta]

This PR conflicts with #12008.