Allow specifying multiple backends for a shell plugin

[hculea]

op CLI version

2.14.0

Goal or desired behavior

Multiple tools that we currently have (or can easily add) plugins for support multiple backends.

Two examples are Docker (DockerHub, Azure/Google/AWS Container Registries etc.) and NPM (official NPM registry, Gitlab/Github package registries etc.).

Current behavior

At this moment, a plugin is not configurable in terms of its backend.

Different tools achieve this in different ways - it may vary between adding a flag/argument to a different credential recipe all together. We should make sure that the solution we put forward is one that abstracts from the third party binary's mechanics.

Relevant log output

No response