Introduce Processable concern for config

What does this MR do?

The Job and Bridge share a lot of configuration. To better align the configuration, the best is to share them via Concern.

This does not change behaviour, just makes it easier to share logic between Bridge and Job (as there's a bunch of disreptencies in behaviour today already), and maps well to Ci::Processable.

I also noticed a few bugs with Ci::Bridge, like when: is completely broken.

I also noticed the misuse of attributes and helpers. We used helpers for entries that were not existing (like for Ci::Bridge).

First for/related to #207484 (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