Configure and use DB distribution URLs

[wagoodman]

Based on #2125 :

• distribution configuration should point to the base URL, excluding the DB schema version and filenames. We want default configuration as well as user configuration to not need to change when bumping the schema versions.
• we should point by default to the new latest.json path (in grype.anchore.io, not toolbox-data.anchore.io)
• we should enable grype db list to reference both the latest.json and history.json files hosted without the user needing to know/configure them (these filenames should not be overridable for now out of simplicity)

Today we have this configuration (with defaults):

db:
  update-url: 'https://toolbox-data.anchore.io/grype/databases/listing.json'

I propose this changes to:

db:
  update-url: 'https://grype.anchore.io/databases'

Where the DB curator (distribution side) object is responsible for:

• using the correct schema version
• looking for the schema appropriate files (latest.json and history.json) when needed