[TASK] Merge indexed_search_mysql

* Merge hook from indexed_search_mysql directly into indexed_search
  and compatibilty7
* Migrate "hook code" merged into indexed_search to doctrine
* Add a feature flag in ext_conf_template "useMysqlFulltext"
* Add a hook to schema migrator if feature flag is enabled to
  register fulltext indexes

Change-Id: I685c16cdcd171257ed1060c4fe0f2b93c4c44ca9
Resolves: #77700
Releases: master
Reviewed-on: https://review.typo3.org/49666


Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>