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Partially addresses #47424.

Now we have a PipelineEntity which is a bit smaller, mostly in bytes
needing to send to the frontend. PipelineDetailsEntity is the default
for the PipelineSerializer, limiting the changes needed.

This commit also incorporates the review.

Added the needed keys and paths to a new entity, BuildDetailsEntity.
Not renaming BuildEntity to BuildBasicEntity on explicit request. Most
code now has test coverage, but not all. This will be added on later
commits on this branch.

Resolves gitlab-org/gitlab-ce#31397