Implement metadata extraction from composer.json files in proxy

[fjmilens3]

Composer projects contain metadata in their composer.json files (https://getcomposer.org/doc/04-schema.md). In other formats we would extract this information when proxying or hosting an artifact, and the information would be included in the format attributes and search attributes.

We are currently not extracting any of this information and should write a general-purpose extractor for this information and store the extracted information. For the proxy implementation this operation should be relatively straightforward since we will only be extracting content from the downloaded source archive rather than a .phar file (which is somewhat different from a .zip or .tar). We'll worry about handling .phar files later if we have to.

Note that this JSON is different than the provider JSON that constitutes an index for purposes of Composer.