Add project_id to pipeline REST api

What does this MR do?

Our Kubernetes agent would like to use the /job api as part of a new feature where it integrates with our CI. Currently it is unable to complete the process because project_id is missing from the API response.

Related issue: #323863 (closed)

Related epic: &5528

This MR will:

• Add project_id to the pipeline entity
• Update associated tests and docs

Does this MR meet the acceptance criteria?

Conformity

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  • I have not included a changelog entry because _____.
• Documentation (if required)
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• Style guides
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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

Related to #323863 (closed)