Implement the Custom Endpoint Plugin

[aaron-congo]

Description

• as custom endpoints are used, monitor threads are created to monitor custom endpoint information
• custom endpoint information is fetched using the RDS API, so users of the CustomEndpointPlugin need to have the boto3 AWS Python SDK
• connections register their plugin services with a given monitor if they are connecting to that monitor's custom endpoint
• when the monitor detects changes in custom endpoint information, it updates the custom endpoint information for all registered plugins
• added new methods in PluginService to get/set allowed/blocked hosts according to custom endpoint information, and to fetch the current list of hosts that are allowed by the custom endpoint plugin. The failover and read/write plugin use this subset of hosts when deciding which host to switch to

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[aaron-congo]

We will hit this block whenever read_only is set to True. Are there performance concerns for this block? It is necessary to protect against the case where we have previously cached a reader instance connection but that reader instance was later removed from the custom endpoint. Without this check the read/write plugin would continue to use the cached reader connection to the now-blocked instance.