Add agent endpoints

[cake-lier]

Context

Currently, Yggdrasil is missing Web API endpoints for accessing agent resources.

Proposed change

Add endpoints for retrieving and manipulating agent resources. This would allow us to represent agents on the platform and then retrieve their representation, which could (at least) show the agent's bodies in the environment the platform supports.

Pros

• Representing agents in the platform allows the relationship between an agent and its bodies to be bidirectional, allowing for crawling starting from an agent URI and then getting all of its bodies instead of starting from a body and understanding which agent is the owner.

Cons

• The agent representations could be redundant, notably if no other motivating use case can be found.
• No use case exists motivating the update of agent representation exists.
• It is unclear when an agent resource should be created or deleted. It could be connected to its authentication in the platform (issue Implement authentication mechanism #41).