[JWT] Support authentication based on JWT

[jsoriano]

Describe the enhancement:

We are going to implement support for authentication based on JWT in Fleet Server. Elastic Agent should prefer the use of JWT when available.

Describe a specific use case for the enhancement or feature:

In scenarios with many agents, having stateless authentication helps to scale.

What is the definition of done?

• [ ] elastic-agent can indicate that it supports JWT on enrollment and obtain tokens instead of API keys.
• [ ] elastic-agent can refresh JWT tokens using access or refresh tokens.
• [ ] elastic-agent can request JWT tokens using an API key. Once JWT token are available, it should forget API keys.
• [ ] elastic-agent can use JWT access tokens for all its operations with Fleet Server.

Related issues

• https://github.com/elastic/fleet-server/issues/3289
• https://github.com/elastic/fleet-server/issues/3290
• https://github.com/elastic/elastic-agent/issues/4278

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)