Implement cli subcommand to install fleet-agent on downstream cluster

[Danil-Grigorev]

In order to install fleet-agent on downstream cluster, a fleet-agent subcommand should be implemented, which will:

• use predefined path to read the token, apiServerURL and apiServerCA from a file (essentially helm values.yaml file)
• perform helm install actions once, according to agent-initiated procedure.

To prevent multiple attempts at installation with equivalent settings on each CP node bootstrap, the command should store its installation state in a Secret. If the secret is not present or contains a diff, a helm upgrade should be performed.