Describe the enhancement:

Elastic Agents have the ability to run in either privileged mode, i.e. with a privileged user like root on Linux systems, or unprivileged mode, i.e. with an unprivileged user.

Moreover, Agents running in privileged mode have the ability to switch themselves to unprivileged mode. This ability is being exposed via the Agent CLI. We now wish to expose this same ability using the Fleet UI.

⚠️ Important note: Fleet UI users should only be switch Agents from privileged to unprivileged mode, not the other way around.

For this, Agent will need to handle a new action from Fleet.

Describe a specific use case for the feature:

Security: Allowing users to reduce the privileges required to run Elastic Agent.

What is the definition of done?

• A new action is defined for switching Agent from privileged to unprivileged mode
• When agent receives said action, it switches from privileged to unprivileged mode
• When the switch succeeds, Agent communicates this success to Fleet
• If the switch fails, Agent communicates this failure to Fleet
• Test exercising the success scenario
• Test exercising the failure scenario