Make it easier to index custom fields in tt_content

[christianbltr]

Currently it is possible to index custom fields added to tt_content by adding two hooks, see https://github.com/teaminmedias-pluswerk/ke_search/blob/master/Documentation/Indexing/IndexerTypes/CustomContentFields.rst

It should be easier to add custom fields since it is a common practice to build custom content elements and these content elements my add fields to tt_content.

A possible implementation could be to add a field in the indexer configuration record where it is possible to add custom fields (like currently "Content element types which should be indexed").