Adds agent-serve-wait-time backend configuration

[c-kruse]

Closes https://github.com/sensu/sensu-go/issues/4825

https://github.com/sensu/sensu-go/issues/4827

What is this change?

• Adds a --agent-serve-wait-time flag to the backend to delay serving agent traffic.
• Adds a /ready API endpoint to agent listener

Why is this change necessary?

Sensu-go backends can be delicate during initial startup in high traffic environments. This allows users to configure a delay after startup before agentd will accept agent traffic.

Does your change need a Changelog entry?

Y

Do you need clarification on anything?

N

Were there any complications while making this change?

N

Have you reviewed and updated the documentation for this change? Is new documentation required?

Big time.

This introduces a new readiness API endpoint and a new backend configuration option. @hillaryfraley

How did you verify this change?

Manual

Is this change a patch?

Not unless it needs to be.