Online DDL/VReplication test suite: support ENUM as part of PRIMARY KEY

[shlomi-noach]

Description

It's valid to have an enum as part of a PRIMARY KEY. In such scenario, we must treat the enum as text and not as integer, to handle a scenario where a query may actually update the enum value (bad practice to update a PRIMARY KEY value, but yet valid).

Added tests:

• enum-pk - tests that enum PK works, that updates to enum (as part of PK) values work
• update-pk - tests that varchar PK works, that updates to varchar (as part of PK) values work

Related Issue(s)

• Tracking: import gh-ost test suite to Vitess/VReplication #8179

Checklist

• Tests were added or are not required
• Documentation was added or is not required

[rohit-nayak-ps]

lgtm