[JWT] Support authentication based on JWT

[jsoriano]

Fleet Server should accept requests by agents authenticated using JWT tokens, as the ones emitted by #3289.

It should validate the JWT token with the following steps:

• The signature of the token, using the public key.
• The expiration time, if present.
• The iss claim must match with the current project.
• The sub claim must match with an agent ID in the .fleet-agents index, to validate that the agent is actually enrolled and not removed from the system. This helps in case of revocation.
• The jti claim must match with one of the current JWT IDs in the .fleet-agents index.

Refresh tokens should not be accepted on any operation, apart on requesting new tokens as defined in #3289.

There must be settings for:

• Controlling the authentication methods supported (API keys, JWT). Default to both.
• Disabling validations on the .fleet-agents index, to make validation completely reliant on stateless JWT. Default to use .fleet-agents for validation.