Add "token_filters" array option to keyword type mapping

[synhershko]

The separation of string to text and keyword is awesome. The only remaining item in my opinion is the not-tokenized-but-lowercased case - it is common enough but will still require some rigorous configuration. It probably makes sense now to allow specifying "token-filters" to execute on "keyword" fields directly in that field's mapping (because different approaches to normalization can be taken - ascii folding, etc). I'm aware of tokenizing tokenfilters (e.g. word-delimiter) but think we can ignore that edge-case for this purpose. WDYT?

[dakrone]

Closing this as a duplicate of #18064 - @synhershko @jpountz is working on this!

[synhershko]

Yay!