Issue #698: Fix roles list keys

[ZCute1]

Description

Fix taf roles list-keys when called without a role argument so it prints each key with its associated role(s), instead of only showing key IDs.

Also added CLI test coverage for taf roles list-keys --path <repo> with no role positional argument.

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Changelog has been updated, as needed (see CHANGELOG.md)

[CLAassistant]

All committers have signed the CLA.

[renatav]

I think is very close to being what we need. My main concern is keeping the public api as intuitive and clean as possible, so it might be a good idea to refactor this a bit before merging.

[renatav]

It was surprising that a similar utility was not already somewhere in the codebase. Let's move this to tuf/repository/MetadataRepository

[renatav]

This works, but is not the most efficient solution. That is fine as we do not expect to ever encounter a situation where we have hundreds of roles.

get_all_roles() walks the full delegation tree and in doing so, it already knows each role's parent, but throws that away and returns a flat list. Then, for each of those role names, get_role_keys(role_name) has to call find_delegated_roles_parent(role). Implementing something similar to find_keysid_roles would be more efficient.

However, this is fine as is, unless you want to tackle the harder implementation as a way of getting more familiar with the system.

[renatav]

I think it's actually better to split the two up:kKeep list_keys_of_role(path, role) exactly as it was (role required), and add a new function for the "all roles" case list_keys_by_roles. This preserves the existing function's contract, and the existing tests should pass without needing any modifications. And then the CLI could call one or the other.