[Documentation]: Add Agent API documentation (versions >= 0.1.3)

[LorenzoTettamanti]

Is your feature request related to a problem?

• Yes

Problem Description:

The latest update has introduced an agent api to connect the CLI (cfcli) with the core components.

Solution:

The documentation needs to be updated to reflect the latest changes. In particular, the explanation of these files is needed:

• API proto file: agent.proto
• API implementation: api.rs