Add node resource availability

[HorjuRares]

Issues: #282

Added a functionality that enables the measurement of node resource availability for each connected agent

Changes:

• modified the internal structure in order to accommodate the agent load
• the agent manager periodically sends info about the available load
• added a new message "AgentLoadStatus"
• added/modified relevant unit tests
• updated the description

[inf17101]

@HorjuRares: I see after approving the last run that 4 stests are failing, maybe due to some reasons known in #363. Maybe you can have a look.

I am not totally finished with the review, but you can still address the comment above.

[inf17101]

Review not finished, but @HorjuRares told me that he will introduce major changes now. So please check if the findings are still matching or otherwise fix them with the new major changes and respond to each of the comment.

[inf17101]

Review finished. Findings published.

[inf17101]

@HorjuRares: We need to check the filtering again. And also the control interface examples.
Try to filter for a certain agent attribute like cpu usage and see if the filtering works.

[krucod3]

Review done. Findings documented.

I saw some requirements that are changed without a version update. Please check and update them.

[krucod3]

I wonder a bit about the space between the value and % and B in the cli output.
@windsource, what do you think?

Otherwise, LGTM 👍

[windsource]

@krucod3 I think there should be no space. Linux commands like ls -lh usually do not print spaces there.