[TASK] Add hook for resolving custom link types

The extendability of the link wizard is incomplete since the
refactoring of it. This patch adds a missing hook to resolve custom
link syntax in the LinkService. It is now possible (again) to
resolve links that start with <identifier>:

Resolves: #79647
Releases: master, 8.7
Change-Id: Iaf0c18f72aa799ab44a8bd14bddd0ca6c3530198
Reviewed-on: https://review.typo3.org/56730


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Reviewed-by: Riny van Tiggelen <info@online-gamer.nl>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>