Use FileStoreMounter in PipelineArtifact

Ref: #234036 (comment 392283242)

What does this MR do?

This MR refactors our file uploader for our PipelineArtifact model.

It does 2 things:

• Use our new module (FileStoreMounter)
• Add spec for direct_upload as suggested in !38221 (comment 395176406)

Why are we doing this?

Following up !37907 (merged) where we introduced a new concern to better encapsulate logic to update the file store column after record creation.

We can now use this new concern within our new model.

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