Allow keyword_marker filter to be updateable

[spinscale]

The keyword_marker filter allows for a keyboard_path configuration setting, but does not seem to support the updateable flag and thus cannot be reloaded via the reload search analyzers API.

To reproduce create a config/analysis/example_word_list.txt file containing

experience
sample

Then run the following in dev-tools

DELETE test_index

PUT /test_index
{
  "settings": {
    "index": {
      "analysis": {
        "analyzer": {
          "search_synonyms": {
            "tokenizer": "whitespace",
            "filter": [
              "my_custom_keyword_marker_filter",
              "porter_stem"
            ]
          }
        },
        "filter": {
          "my_custom_keyword_marker_filter": {
            "type": "keyword_marker",
            "keywords_path": "analysis/example_word_list.txt",
            "updateable": true
          }
        }
      }
    }
  }
}

# cat example_word_list.txt
# experience 
# sample

# sample and experience are untouched
GET test_index/_analyze
{
"text": "this is a experience sample",
"analyzer": "search_synonyms"
}

# Now remove the `sample` line from the file and reload
POST /test_index/_reload_search_analyzers

# retry, sample should be `sampl` now if updateable, but remains sample
GET test_index/_analyze
{
"text": "this is a experience sample",
"analyzer": "search_synonyms"
}

Another issue with this is, that anything can be configured in my_custom_keyword_marker_filter and thus setting/believing the updateable parameters works can be misleading.

[elasticmachine]

Pinging @elastic/es-search (Team:Search)

[cbuescher]

I can see the benefit of being able to update the keyword_marker list used for search analyzers without node shutdown.
In many ways this resource file is comparable to a synonym list. However it would be the first non-synonym filter that would be
updateable so I think this request needs some more inspection and discussion. As far as implementation goes it should be sufficient
to wire the "updateable" setting in the same way as we do for synonym filters.