Refactor: New operator InsertionSelection to adhere to the operator model

[GuptaManan100]

Description

In the operator model that we have, a Route represents a part of the operator that should be converted to a query and run as a single unit. It therefore doesn't make sense to have a Route operator nested inside another.

Previously for a query like insert into music(id, user_id) select * from user, the operator tree looked like -

Route (Scatter on user Seen:[<nil>])
└── Insert (user.music)
    └── Route (Scatter on user Seen:[<nil>])
        └── Horizon
            └── Table (user.user)

This refactor introduces a new operator called InsertionSelection to remedy this situation.

After the refactor the operator tree looks like -

InsertSelection
├── Route (Scatter on user Seen:[<nil>])
│   └── Horizon
│       └── Table (user.user)
└── Route (Scatter on user Seen:[<nil>])
    └── Insert (user.music)

Related Issue(s)

Tracking issue #11626

Checklist

• "Backport to:" labels have been added if this change should be back-ported
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Deployment Notes

[vitess-bot]

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[arthurschreiber]

👍