Capture job executor value in ci_runners table

What does this MR do and why?

Describe in detail what your merge request does and why.

bin/rails db:migrate RAILS_ENV=development

== 20211208111425 AddExecutorTypeColumnToCiRunners: migrating =================
-- add_column(:ci_runners, :executor_type, :smallint, {:null=>true})
   -> 0.0036s
== 20211208111425 AddExecutorTypeColumnToCiRunners: migrated (0.0037s) ========

bin/rails db:rollback RAILS_ENV=development

== 20211208111425 AddExecutorTypeColumnToCiRunners: reverting =====================
-- remove_column(:ci_runners, :executor_type, :smallint, {:null=>true})
   -> 0.0023s
== 20211208111425 AddExecutorTypeColumnToCiRunners: reverted (0.0045s) ============

Screenshots or screen recordings

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The second query shows the ci_runners record after being populated by the /jobs/request endpoint hit by the GitLab Runner.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Run migration: bin/rails db:migrate RAILS_ENV=development

2. Register a runner against the local GDK:

   gitlab-runner register -config /tmp/config.gdk.toml \
           --non-interactive \
           --executor "shell" \
           --url "http://gdk.localhost:3000/" \
           --description "Gitlab Org test runner" \
           --tag-list "shell,gdk,mac,test" \
           --run-untagged="false" \
           --locked="false" \
           --access-level="not_protected" \
           --registration-token="$GITLAB_TOKEN"

3. Start the runner:

   gitlab-runner run -config /tmp/config.gdk.toml

4. Open a Rails console: rails c

5. Query for the executor on the last-registered runner:

   Ci::Runner.select(:id, :executor_type).last

   Expected result:

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.

Part of #345361 (closed)