Parse enum/set values with sqlparser

[shlomi-noach]

Description

This is a minor refactor/cleanup PR, followup to #15493.

https://github.com/vitessio/vitess/pull/15493/files#diff-e6d36f19ff8893ef05db3d0b9297a87ed3a0ecb6cdf7486c93718344b2ac25f9 introduced a CSV-style parsing to the enum set (e.g. 'x-small','small','medium','large','x-large'). In this PR we use sqlparser to parse these values. sqlparser cannot directly parse this, so we wrap around with a dummy query:

alter table t add column e enum('x-small','small','medium','large','x-large')

We then parse the complete query. This is more wasteful, of course, as we need to parse a statement, cast to AlterTable, extract AddColumns, and finally read EnumValues. Nonetheless, this utilizes our de-facto parser and I think it's worth the overhead for the little use case this gets.

Related Issue(s)

#15492

Checklist

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[shlomi-noach]

EDIT: this comment became irrelevant thanks to 02b6c9b

---

It is worth pointing out that escaped values are outputted differently:

input := `'with '' quote','and \n newline'`

Previously:

expect := []string{"with ' quote", "and \n newline"}

In this PR:

expect := []string{"with \\' quote", "and \\n newline"}

[dbussink]

Like that this uses the proper parser now.

[codecov]

Codecov Report

Attention: Patch coverage is 70.00000% with 15 lines in your changes missing coverage. Please review.

Project coverage is 67.44%. Comparing base (fdf529e) to head (270c0d5).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/schema/parser.go	70.27%	11 Missing ⚠️
go/vt/vttablet/tabletserver/vstreamer/vstreamer.go	69.23%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17133      +/-   ##
==========================================
+ Coverage   67.42%   67.44%   +0.01%     
==========================================
  Files        1571     1569       -2     
  Lines      252184   252109      -75     
==========================================
- Hits       170043   170032      -11     
+ Misses      82141    82077      -64     

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[shlomi-noach]

I pushed a small change that uses sqltypes.DecodeStringSQL to decode the enum values strings, which is even more formal now, and also happens to agree with the tests from before this PR, so that #17133 (comment) becomes irrelevant.