VReplication: Support both vindex col def formats when initing target sequences

[mattlord]

Description

You can specify the columns which make up a vindex in two ways within the vschema:

1. The "legacy" way (a simple string):

   "corder": {
     "column_vindexes": [
       {
         "column": "customer_id",
         "name": "hash"
       }
     ],
     "auto_increment": {
       "column": "order_id",
       "sequence": "order_seq"
     }
   },

2. The "new" way (an array):

   "product": {
     "column_vindexes": [
       {
         "columns": [
           "product_id"
         ],
         "name": "hash"
       }
     ],
     "auto_increment": {
       "column": "product_id",
       "sequence": "product_seq"
     }
   }

The MoveTables target sequence initialization work (added in https://github.com/vitessio/vitess/pull/13656) only supports the legacy way. It needs to be updated to support either.

That's exactly what this PR does. 🙂

I think that we should backport this to v18 because:

1. That's where this feature was first added via https://github.com/vitessio/vitess/pull/13656
2. It's a very small and safe change

Related Issue(s)

• Fixes: https://github.com/vitessio/vitess/issues/16861

Checklist

• [x] "Backport to:" labels have been added if this change should be back-ported to release branches
• [x] If this change is to be back-ported to previous releases, a justification is included in the PR description
• [x] Tests were added or are not required
• [x] Did the new or modified tests pass consistently locally and on CI?
• [x] Documentation was added or is not required

[vitess-bot[bot]]

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.00000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 69.44%. Comparing base (2e2b223) to head (ab83107). Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtctl/workflow/traffic_switcher.go	75.00%	5 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #16862 +/- ## ========================================== + Coverage 69.43% 69.44% +0.01% ========================================== Files 1571 1571 Lines 203021 203095 +74 ========================================== + Hits 140970 141042 +72 - Misses 62051 62053 +2 ```

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