Add API functions to retrieve active instances and their devices

[zacikpa]

Resolves: RHEL-15137

Appropriate commands are added to tuned-adm as well:

• get_instances: Retrieves a list of active instances of a given plugin or all active instances.
• instance_get_devices: Retrieves a list of devices assigned to a given instance.

To differentiate between cases when a plugin does not exist (error) and when it does not have any active instances (not an error), get_instances also returns a boolean and an error string.

Similarly in instance_get_devices - if an instance exists but does not have any assigned devices, it is not an error. If it does not exist, it is an error.

Let me know if this behaviour is OK or whether I should simplify it by just returning empty lists.

[zacikpa]

@yarda Please review the updates and I will squash then.

[yarda]

EPEL-8 CI failure is infra error, i.e. it is not blocking the merge.

[yarda]

LGTM, please squash.

[zacikpa]

LGTM, please squash.

Done.