Add Maven registry support

[markelliot]

Fixes #284

Add a Maven registry backend for buffrs:

• Extract a Registry trait
• Encode registry type as a prefix in RegistryUri
• Dispatch to the correct Registry implementation from RegistryUri

[mara-schulke]

Overall im very happy with the code quality and the implementation approach of this pr! Thank you! 😊

My two dots:

1. This PR is missing tests, before merging i'd like to have end to end tests that ensure that: mixed maven and artifactory setups, maven publishing and maven installation are working as expected. You might need to change the e2e test setup slightly to account for having two registries.
2. Left some comments on type organisation and the possibility to use a builder pattern. Would prefer if they get addressed but it's not a blocker to merging if you feel strongly about the current implementation.