Return just latest versions in proxy mode /search

[mrodm]

This PR adds the needed changes to return just the latest versions of each package in queries against /search. Related to #849

Scenario:

• In development:
  • elastic_package_registry version 0.0.4
  • elastic_package_registry version 0.0.5
  • elastic_package_registry version 0.0.6
• In EPR proxy:
  • Up to 0.0.5 version published of elastic_package_registry

Tested using elastic-package stack up

• Before:

 $ curl -k -s "https://localhost:8080/search?experimental=true&package=elastic_package_registry" | jq -r .[].version
0.0.5
0.0.6

• After:

 $ curl -k -s "https://localhost:8080/search?experimental=true&package=elastic_package_registry" | jq -r .[].version
0.0.6

[elasticmachine]

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• Start Time: 2022-09-09T13:59:32.823+0000

• Duration: 5 min 53 sec

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Test	Results
Failed	0
Passed	213
Skipped	0
Total	213

🤖 GitHub comments

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[mrodm]

As there has not been any release with the proxy mode enabled , I've just added this PR here

[mtojek]

Thank you for fixing this!