Add parsing support to := operator

[Ayman161803]

Description

This PR adds support to assignment operator := to parse queries of the type mentioned in #10760.
The MySQL docs can be found here - https://dev.mysql.com/doc/refman/8.0/en/assignment-operators.html

Related Issue(s)

#10760
#8604

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

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[GuptaManan100]

@vitessio/query-serving This is good from @Ayman161803. The current changes adds the parsing support for the assignment operator, but I tested out our planner, it doesn't handle it well. The planner needs to realize that an assigment operator is basically assigning the user defined variable and then using it, so we need to plan it as a set and then whatever the command is.

[GuptaManan100]

I have explicitly disallowed this expression in the planner for now.