[Fleet] Support fetching single asset file contents

In EPR v1, we supported fetching a single file's contents via EPR. We'd like to add support for this behavior back in EPR v2 to support fetching individual assets even when an integration hasn't been installed in a given Kibana instance.

e.g. https://epr.elastic.co/package/security_detection_engine/8.7.1/kibana/security_rule/000047bb-b27a-47ec-8b62-ef1a5d2c9e19.json should respond with the JSON content contained in this file.

These routes can return `content-type: text/plain` and don't need to honor the given file's MIME type.

Start by tracing the EPR v2 route handling for the example asset URL and compare it with the EPR v1 behavior described in the issue. The work is done when an individual asset URL returns that file's contents with content-type: text/plain, including when its integration is not installed.

Written by the indexing model from the issue text.