add able to use Proc for settings

[parallel588]

Hi Arkadiy (@pyromaniac), I added test.
about case where it may be use:
if we keep a synonyms/stopwords in db then in the settings we can use synonyms from db
for instance:

 class ProductsIndex < Chewy::Index
  settings proc {
    {
      analysis: {
        filter: {
          synonym: {
            type: 'synonym',
            ignore_case: true,
            synonyms: SearchSynonym.synonyms
          },
          
          custom_stop: {
            type: 'stop',
            stopwords: SearchStopWord.terms
          },
.....

and we can update settings after changes SearchSynonym.synonyms

   Chewy.client.indices.put_settings(
        index: productsIndex.index_name,
        body: {
          index: {
            analysis: {
              filter: {
                synonym: {
                  synonyms: SearchSynonym.synonyms
                }
              } } } }
      )

[parallel588]

Hi @pyromaniac , I mistakenly closed the previous PR#440
Could you pleas check it PR?

[pyromaniac]

Ah, here you are!

[pyromaniac]

Can you use stub_const here instead please?

[pyromaniac]

Would appreciate if you will do a small fix, and I'll merge it right away, thanks!

[pyromaniac]

Great, thanks!