Add support for enabling and disabling Agents

[william20111]

Details

As an developer using this library I would like to disable & enable agents via library

AC

• Add support for agent disable on the Agent struct
• Add support for agent disable on the Agent struct
• Add tests to cover new funcs

References

• https://developer.thousandeyes.com/v6/endpoint_agents/#/endpoint_agent_enable
• https://developer.thousandeyes.com/v6/endpoint_agents/#/endpoint_agent_disable