Update metadata records based on ActivityPub server push

[pombredanne]

In a decentralized scenario, there are two or more servers cooperating. Here I would like to start building this multiserver architecture. Specifically, we should build an application to run in a second VulnerableCode and/or PurlDB server that can receive updates notifications from the FederatedCode ActivityPub server for a package and then fetch the data from the backing git stores. And use this to update this separate PurlDB instance, including minimally reconciling conflicts.
This could include:

• Create the function(s)/services to receive updates notifications from the FederatedCode ActivityPub server in PurlDB Function(s) and scripts to discover and retrieve existing Packages existing in the AP server(s). The input is always a PURL or list of PURLs. #51
• Create script(s) to fetch the backing data from the corresponding FederatedCode backing Git repository and update the local records in PurlDB, reconciling conflicts as needed Function(s) and scripts for PurlDB to subscribe to Package updates over AP #47

For a related future step for VulnerableCode see:

• Update VulnerableCode on AP server PUSH  #57

[keshav-space]

Completed in: #52, #49, #50 and aboutcode-org/purldb#557

• Function(s) and scripts to discover and retrieve existing Packages existing in the AP server(s). The input is always a PURL or list of PURLs. #51

To test this follow the steps here #51 (comment)

• Function(s) and scripts for VulnerableCode and/or PurlDB to subscribe to Package updates over AP #47

To test this follow the steps here #47 (comment)