[TASK] Move table definition merge to `SchemaMigrator`

To give extensions the ability to change the expected
database schema, adding or changing columns of existing
TYPO3 Core tables or other extension tables,
`ext_tables.sql` of extension are read and taken as partial
table definitions.

All table definitions are merged at a later point in the
`ConnectionMigrator`, forcing proper handling of partial
table information.

This change moves the table definition merge process from
the `ConnectionMigrator` to the `SchemaMigrator` at the
earliest possible place to pave the way for further
code cleanups.

Resolves: #103015
Related: #103014
Releases: main
Change-Id: I7f5bfa6f438604ca6295f20eb85fb9e24a44189a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82758


Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni Mack <benni@typo3.org>