Static proxy with metadata cache

[akondas]

This is the thread where all changes related to the static proxy will be summarized. We give up the solution that downloads metadata files on the fly (no possibility of proper caching).

• remove MetadataProvider and DistStorage from proxy (Serve static proxy metadata and use v2 endpoint for dist lookup #222)
• return cache headers in metadata endpoints (Serve static proxy metadata and use v2 endpoint for dist lookup #222)
• synchronize metadata files (cli command + cron) (Implement sync proxy metadata command #224)
• make download return stream instead download file to memory (Use async and stream for downloading metadata and distributions files #226)
• use composer v2 endpoints to lookup for distribution (smaller files) (Serve static proxy metadata and use v2 endpoint for dist lookup #222)
• replace all file functions with filesystem (this will be base for add support for using S3 as storage, thanks @jonnyynnoj for inspiration) (Move all proxy logic to Proxy class #223)
• separate proxy logic from private packages (Proxy class will encapsulate all) (Move all proxy logic to Proxy class #223)
• create cli command for renaming dist files in proxy dir (Create migration for better auto upgrade to 0.5.0 #227)

⚠️ Unfortunately, there will be slight changes that cause a little incompatibility:

• metadata files now will be full json files (previously it was serialized php array)
• dist file will be saved only with reference in name (to reduce storage size)

Upgrading to 0.5.0:

• if you are using migrations (you should) then noting to do 😉 (thanks to Create migration for better auto upgrade to 0.5.0 #227)