Add planner-version flag to vtexplain

[mattlord]

Description

With Vitess 12.0 we hope to begin migrating more users and workloads from the V3 planner to the new Gen4 planner as it should offer better performance, improved correctness, and additional query language support.

But in order to aid in that effort, users will need a way to try and compare the planner results between the two for their common queries in order to gauge the impact of this migration.

Toward that end, this adds a new -planner-version flag to vtexplain that allows you to get the explain output using the Gen4 planner instead of using the default of V3.

Related Issue(s)

Docs PR: vitessio/website#843

Manual Test/Example

## Invalid version
$ vtexplain -planner-version=Gen5 -shards 2 -schema-file /tmp/schema -vschema-file=/tmp/vschema -sql 'SELECT MAX(co.price) FROM customer c JOIN corder co ON c.customer_id = co.customer_id WHERE c.email != "mlord@planetscale.com" AND co.sku = "foobar" AND co.order_id = 0 GROUP BY co.price'
ERROR: invalid value specified for planner-version of 'Gen5' -- valid values are V3 and Gen4

## V3
$ vtexplain -planner-version=V3 -shards 2 -schema-file /tmp/schema -vschema-file=/tmp/vschema -sql 'SELECT MAX(co.price) FROM customer c JOIN corder co ON c.customer_id = co.customer_id WHERE c.email != "mlord@planetscale.com" AND co.sku = "foobar" AND co.order_id = 0 GROUP BY co.price'
ERROR: vtexplain execute error in 'SELECT MAX(co.price) FROM customer c JOIN corder co ON c.customer_id = co.customer_id WHERE c.email != "mlord@planetscale.com" AND co.sku = "foobar" AND co.order_id = 0 GROUP BY co.price': unsupported: in scatter query: group by column must reference column in SELECT list
1 commerce/-: select c.customer_id from (select customer.customer_id from customer where customer.customer_id = 5) as c join corder on c.customer_id = corder.customer_id limit 10001

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## Gen4
$ vtexplain -planner-version=Gen4 -shards 2 -schema-file /tmp/schema -vschema-file=/tmp/vschema -sql 'SELECT MAX(co.price) FROM customer c JOIN corder co ON c.customer_id = co.customer_id WHERE c.email != "mlord@planetscale.com" AND co.sku = "foobar" AND co.order_id = 0 GROUP BY co.price'
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SELECT MAX(co.price) FROM customer c JOIN corder co ON c.customer_id = co.customer_id WHERE c.email != "mlord@planetscale.com" AND co.sku = "foobar" AND co.order_id = 0 GROUP BY co.price

1 commerce/-80: select MAX(co.price), co.price, weight_string(co.price) from customer as c, corder as co where c.email != 'mlord@planetscale.com' and co.sku = 'foobar' and co.order_id = 0 and c.customer_id = co.customer_id group by co.price, weight_string(co.price) order by co.price asc limit 10001
1 commerce/80-: select MAX(co.price), co.price, weight_string(co.price) from customer as c, corder as co where c.email != 'mlord@planetscale.com' and co.sku = 'foobar' and co.order_id = 0 and c.customer_id = co.customer_id group by co.price, weight_string(co.price) order by co.price asc limit 10001

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Checklist

• Should this PR be backported? Yes: release-12.0
• Tests were added or are not required
• Documentation Document VTExplain planner-version flag website#843