[Fleet] Allow to specify kibana.version when fetching categories

[nchaulet]

Description

Related to elastic/kibana#94582

The categories count in Kibana is sometimes invalid has some package could not exists depending on the kibana version.

This PR fix it by allowing to pass the kibana.version (same parameter name used in the search handler) query params to the category handler, and filtering package that have the correct kibana version.

Examples

➜  ~ http http://localhost:8080/categories\?experimental\=true\&kibana.version\=7.11.0 | jq '.[] | select(.id =="elastic_stack")'
{
  "id": "elastic_stack",
  "title": "Elastic Stack",
  "count": 1
}
➜  ~ http http://localhost:8080/categories\?experimental\=true | jq '.[] | select(.id =="elastic_stack")'
{
  "id": "elastic_stack",
  "title": "Elastic Stack",
  "count": 3
}
``

[elasticmachine]

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• Build Cause: Pull request #695 updated

• Start Time: 2021-05-11T12:40:00.971+0000

• Duration: 7 min 42 sec

• Commit: 1db74ed

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[mtojek]

Thank you for working on the fix, nice finding BTW.

Would you mind covering relevant endpoints with unit tests? See: https://github.com/elastic/package-registry/blob/master/main_test.go#L32

Please also update the CHANGELOG file.

[mtojek]

LGTM!

Speaking of releasing this change, here is the guide: https://github.com/elastic/package-registry/#release