Allow to use a custom Highlighter

[gwenzek]

In Nest 2.3.2 the HighlightField uses an enum HighlighterType as Type.

This prevents to use a custom highlighter instead of one of the 3 embedded within ElasticSearch.

This is a regression from Nest 1.7 where the same field was a String, therefore allowing to put the name of an highlighter provided by a plugin.

I think having a string as Type field and a few const string inside a static HighlighterType class would give the best of both world.

[gmarz]

Similar to #2119. Labeling for 5.0 as this would be a breaking change. Thanks for reporting @gwenzek !

[Mpdreamz]

Hi @gwenzek we found a way to include support for this in 2.x without introducing a breaking change so a fix for this should go into the next release of 2.x