Allow REST API search and download through proxy repositories

Currently a REST API search or download request against a proxy repository will only work if the components are cached locally in a proxy repository. 

For instance a request to download the latest snapshot jar for a GAV:

curl "http://localhost:8083/service/rest/v1/search/assets/download?sort=version&repository=maven-snapshots-proxy&maven.groupId=org.foo.bar&maven.artifactId=project&maven.baseVersion=1.2.3-SNAPSHOT&maven.extension=jar"

This will fail unless the jar is already downloaded into the proxy.

Retrieving the latest version of a component via REST API through a proxy is a use case we should be supporting.  Possible implementations for this include implementing a remote search capability, or, specifically for Maven, using the maven-metadata.xml files to find available versions.