Add code suggestions models

What does this MR do and why?

Related to #423522 (closed)

• adds basic structure for code suggestion models which allows using different model for different code creation inputs (currently only from comment) - #423522 (comment 1534934962)
• adds prompt parameter to code suggestion which wraps original "prefix" in markdown
• adds prompt_version parameter - 1 is used by default, 2 for generation endpoint when prompt is added (gitlab-org/modelops/applied-ml/code-suggestions/ai-assist#284 (comment 1541997759))

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

(optional - for testing with AI_gateway) point Rails to use your local AI gateway by setting CODE_SUGGESTIONS_BASE_URL and restart rails-web

$ cat ../env.runit
export GITLAB_SIMULATE_SAAS=1
export CODE_SUGGESTIONS_BASE_URL=http://0.0.0.0:5052

For checking outgoing params, you can log the content of the request passed to workhorse, something like:

diff --git a/ee/lib/api/code_suggestions.rb b/ee/lib/api/code_suggestions.rb
index 0a07dbedec23..ec5f07586362 100644
--- a/ee/lib/api/code_suggestions.rb
+++ b/ee/lib/api/code_suggestions.rb
@@ -117,6 +118,7 @@ def gitlab_realm
             params: params.except(:private_token)
           )
 
+          Rails.logger.error model.body
           workhorse_headers =
             Gitlab::Workhorse.send_url(
               model.endpoint,

and then check with curl request that the outgoing request contains prompt and prompt_version params.

curl --header "PRIVATE-TOKEN: xyz" -d 'current_file[file_name]=test.py&current_file[content_above_cursor]=// GitLab Duo Generate: write hello world function &current_file[content_below_cursor]=aaaa' --request POST http://192.168.1.8:3000/api/v4/code_suggestions/completions

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.