[Fleet] Support fetching single asset file contents

[kpollich]

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.