Generate JWT for authentication and provide it to CI jobs

What does this MR do?

Generate JWT that can be used for authentication with 3rd parties that support JWT authentication (e.g. Vault - https://www.vaultproject.io/docs/auth/jwt/#jwt-authentication) and provide it to jobs as CI_JOB_JWT predefined CI variable.

See #207125 (closed).

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

app/models/ci/build.rb

@@ -526,6 +526,7 @@ def variables
       strong_memoize(:variables) do
         Gitlab::Ci::Variables::Collection.new
           .concat(persisted_variables)
+          .tap {|variables| variables.append(ci_job_jwt) if Feature.enabled?(:ci_job_jwt, project, default_enabled: true)}
           .concat(scoped_variables)
           .concat(job_variables)
           .concat(environment_changed_page_variables)
@@ -981,6 +982,15 @@ def options_retry
     def has_expiring_artifacts?
       artifacts_expire_at.present? && artifacts_expire_at > Time.now
     end
+
+    def ci_job_jwt
+      {
+        key: 'CI_JOB_JWT',
+        value: Gitlab::Ci::Jwt.for_build(self),
+        public: false,
+        masked: true
+      }
+    end
   end
 end