Allow CI bridge to expand variables that use pipeline ID and URL

What does this MR do?

Allows using $CI_PIPELINE_ID and $CI_PIPELINE_URL when defining variables for downstream pipelines in bridge jobs.
Example:

remote:
  variables:
    UPSTREAM_CI_PIPELINE_ID: "${CI_PIPELINE_ID}"
    UPSTREAM_CI_PIPELINE_URL: "${CI_PIPELINE_URL}"
  trigger: some/project

The pipeline in some/project will receive $UPSTREAM_CI_PIPELINE_ID and $UPSTREAM_CI_PIPELINE_URL expanded to the values from the current pipeline.

Related issue #13070 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• 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

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