[Fleet] Add categories to search response output

[thomasneirynck]

Closes #722

This adds the categories to the search-response output.

• Inclusion of categories is required so clients can build a facetted browser, merging multiple sources into overlapping category taxonomy. (this is the goal of [Fleet] Unified Integrations view kibana#93084)
• Adding this will also avoid having to run repeated requests to EPR. Now Kibana needs to issue a search each time a category is selected. It also needs to run a single-search up-front for all packages just so it can provide a total count. This PR will remove the need for this redundant network traffic.

cc @joshdover

[elasticmachine]

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• Start Time: 2021-09-28T19:05:51.721+0000

• Duration: 5 min 38 sec

• Commit: d2bbfb6

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[mtojek]

Thanks for the PR. Let's make sure if it works correctly for all our use cases.

1. Please run make formatmage format to clean this code.
2. Please add a changelog entry to the CHANGELOG.md.
3. Please run go test . to see if something has changed (-generate to regenerate golden files).

[mtojek]

LGTM!

I double-checked it against the policy template case (url: /search?category=datastore&experimental=true) and it seems to be correct: https://github.com/elastic/package-registry/blob/d2bbfb6efbca8c93d0c9195c871d40450817fae0/testdata/generated/search-category-datastore.json

I think you're good to go, Thomas. You can proceed with releasing procedure. If you encounter any blockers, we can help you with them. FYI release cycles of EPR are detached from Elastic release trains, so feel free to do this anytime.