server: fix resultType/flag of enum&set column

[lysu]

What problem does this PR solve?

fixes #6910

In MySQL-protocol for enum and set type, server send column-metadata as string type and remark enum or set in flag field.

maybe see:
https://github.com/mysql/mysql-server/blob/99a4c9262047edce8bb67086d22f745c67afe2ef/sql/field.h#L4165

https://github.com/mysql/mysql-server/blob/99a4c9262047edce8bb67086d22f745c67afe2ef/sql/field.h#L4225

What is changed and how it works?

modify ColumnInfo#Dump()(write metadata stage) to return special type, and doesn't modify column.type, because later we will use it to write data stage.

Check List

• Integration test, in TiDB-Test with protocol in https://github.com/pingcap/tidb-test/pull/603

Code changes

• Change response

Side effects

• no

Related changes

• Need to cherry-pick to the release branch
• Need to be included in the release note

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This change is 

[zz-jason]

does type bit have the same issue?

[lysu]

/run-all-tests tidb-test=pr/603

[lysu]

it seems bit is ok and no magic, only enum/set return a different type - -|||

[zz-jason]

OK, LGTM

[coocood]

LGTM