Uses newest scoring timestamp as etag header

[alecharp]

Description

Closes #373.

Not all scores are computed all the time. For a score to be computed again on a plugin, probe results on the plugin have to be updated (more recent than last execution of the score) and the scoring implementation must have the same version as on its last execution.

Because of that, we know that the most recent score computed, across all the plugins, will determine if the content of the /scores/api has changed since the last time it was fetched.

This puts the value of the seconds since epoch of the most recent scoring execution in the ETag header of the response.

Testing done

Adjusted the unit test on the API to validate the presence and value of the ETag header.

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1. If an issue exists, it is well described and linked in the description
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3. The changeset is on a specific branch. Using feature/ for new feature, or improvements ; Using fix/ for bug fixes ; Using docs/ for any documentation changes.
4. If required, the documentation has been updated
5. There is automated tests to cover the code change / addition or an explanation why there is no tests in the description.

[alecharp]

@lemeurherve is that enough or should we have something in front of the /scores/api interface to cache the response and "keep" the data as long as it's not changed?

[lemeurherve]

I don't know.

[lemeurherve]

I definitively would be in favor of a static export of everything like it's done for other sources of data for plugin-site instead of an API, but that's a discussion not directly related to this PR.

[lemeurherve]

I definitively would be in favor of a static export of everything like it's done for other sources of data for plugin-site instead of an API, but that's a discussion not directly related to this PR.

Looking again at the linked issue, that's in fact related.

#373 is about having a static export in https://reports.jenkins.io instead of an API to pass data to plugin-site.

don't resolve the issue as I intended initially