Add new feature flag type called `worker`

What does this MR do and why?

Add new feature flag type called worker

This feature flag type is similar to undefined where the feature flag names are not defined in the GitLab codebase. Each feature flag's name might have the worker name in it. These feature flags will be used to control the behavior of Sidekiq workers dynamically, e.g. deferring jobs from runaway workers during an incident.

Changelog: added

---

Resolves gitlab-com/gl-infra/scalability#2345.

More context can be found in the epic gitlab-com/gl-infra&1004 (closed) and this discussion.

The immediate usage of this new type of feature flag is at Defer Sidekiq jobs from worker type feature flags (!120606 - merged)

How to set up and validate locally

1. Checking for a random feature flag will follow the default_enabled_if_undefined behavior

   [1] pry(main)> Feature.enabled?(:some_feature_flag, type: :worker, default_enabled_if_undefined: false)
     Feature::FlipperGate Load (0.3ms)  SELECT "feature_gates".* FROM "feature_gates" WHERE "feature_gates"."feature_key" = 'some_feature_flag' /*application:console,db_config_name:main,console_hostname:Gregoriuss-MBP,console_username:gregoriusmarco,line:/lib/feature.rb:237:in `block in current_feature_value'*/
   => false
   [2] pry(main)> Feature.enabled?(:some_feature_flag, type: :worker, default_enabled_if_undefined: true)
   => true

2. Turn on that feature flag from API (requires to create a personal access token first):

   $ curl -s -H "PRIVATE-TOKEN: $PERSONAL_ACCESS_TOKEN" http://gdk.test:3000/api/v4/features/some_feature_flag --data "value=true" | jq
   {
     "name": "some_feature_flag",
     "state": "on",
     "gates": [
       {
         "key": "boolean",
         "value": true
       }
     ],
     "definition": null
   }

3. Check again if the feature is enabled:

   [4] pry(main)> Feature.enabled?(:some_feature_flag, type: :worker, default_enabled_if_undefined: false)
   => true
   [5] pry(main)> Feature.enabled?(:some_feature_flag, type: :worker, default_enabled_if_undefined: true)
   => true

MR acceptance checklist

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