Pass hash instead of URI to Elasticsearch client

What does this MR do and why?

We have issues when there are special characters in Elasticsearch credentials. Historically, we passed a URI to the elasticsearch client, but that forces us to url encode the credentials. This MR changes the Elasticsearch connection details to be passed to elasticsearch client as a hash to avoid this problem.

It's worth noting that the elasticsearch client gem uses a raw hash anyways: https://github.com/elastic/elasticsearch-ruby/blob/v7.3.0/elasticsearch-transport/lib/elasticsearch/transport/client.rb#L196

Additionally, this doesn't put any additional guard rails around passing credentials with special characters in elasticsearch_url application setting. Users still need to pass a URI safe url that won't raise an error in URI.parse. For example, this still will not work:

[1] pry(main)> ApplicationSetting.current.update(elasticsearch_url: 'http://user:p@ssword@localhost:9200/')
URI::InvalidURIError: bad URI(is not URI?): "http://user:p@ssword@localhost:9200"

It is expected that the user either a) percent-encode the credentials in the URL or b) use raw, unencoded values in elasticsearch_username and elasticsearch_password application settings:

[2] pry(main)> ApplicationSetting.current.update(elasticsearch_url: 'http://user:p%40ssword@localhost:9200/', elasticsearch_username: nil, elasticsearch_password: nil)
=> true
[3] pry(main)> ApplicationSetting.current_without_cache.elasticsearch_url_with_credentials
=> [{:scheme=>"http", :user=>"user", :password=>"p@ssword", :host=>"localhost", :path=>"", :port=>9200}]
[4] pry(main)> ApplicationSetting.current.update(elasticsearch_url: 'http://localhost:9200/', elasticsearch_username: 'user', elasticsearch_password: 'newp@ssword')
=> true
[5] pry(main)> ApplicationSetting.current_without_cache.elasticsearch_url_with_credentials
=> [{:scheme=>"http", :user=>"user", :password=>"newp@ssword", :host=>"localhost", :path=>"", :port=>9200}]

Related to: #13739 (closed) Closes: #354456 (closed)

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How to set up and validate locally

1. If you have gdk Elasticsearch running, do gdk stop elasticsearch
2. Start an elasticsearch container with password that includes a special character:

docker run --rm -p 9200:9200 -e "discovery.type=single-node" -e 'xpack.security.enabled=true' -e 'ELASTIC_PASSWORD=p@ssword' docker.elastic.co/elasticsearch/elasticsearch:7.16.1

3. In a rails console, update the advanced search settings

ApplicationSetting.current.update(elasticsearch_url: 'http://localhost:9200/', elasticsearch_username: 'elastic', elasticsearch_password: 'p@ssword')

4. Run indexing rake task: bundle exec rake gitlab:elastic:index. This is asynchronous and may take a bit of time to complete even after command exits successfully.
5. Verify that code was indexed successfully by visiting http://localhost:9200/_search?q=flight

MR acceptance checklist

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