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

.gitlab/issue_templates/Feature Flag Roll Out.md

@@ -37,7 +37,7 @@ If applicable, any groups/projects that are happy to have this feature turned on
 - [ ] Coordinate a time to enable the flag with `#production` and `#g_delivery` on slack.
 - [ ] Announce on the issue an estimated time this will be enabled on GitLab.com
 - [ ] Enable on GitLab.com by running chatops command in `#production`
-- [ ] Cross post chatops slack command to `#support_gitlab-com` and in your team channel
+- [ ] Cross post chatops slack command to `#support_gitlab-com` ([more guidance when this is necessary in the dev docs](https://docs.gitlab.com/ee/development/feature_flags/controls.html#where-to-run-commands)) and in your team channel
 - [ ] Announce on the issue that the flag has been enabled
 - [ ] Remove feature flag and add changelog entry
 - [ ] After the flag removal is deployed, [clean up the feature flag](https://docs.gitlab.com/ee/development/feature_flags/controls.html#cleaning-up) by running chatops command in `#production` channel