[ENHANCEMENT] Table: improve migration of column settings

[AntoineThebaud]

Description

• Improve the logic of the Table migration schema to merge settings related to the same column in a single object (not 100% perfect but still better than before, see test data modifications). As a reminder, at render time the Table panel only considers the first columnSetting it finds corresponding to a given column, thus further settings aplying to the same column are simply ignored, even if they tune different attributes.
• small improvement in the migration of legacy table panel

As a good side effect it solves perses/perses#3701.

Checklist

• Pull request has a descriptive title and context useful to a reviewer.
• Pull request title follows the [<catalog_entry>] <commit message> naming convention using one of the
  following catalog_entry values: FEATURE, ENHANCEMENT, BUGFIX, BREAKINGCHANGE, DOC,IGNORE.
• All commits have DCO signoffs.