Add Validate RPC to the Config Service

[edwbuck]

This feature adds a new RPC call to validate a plugin configuration without the plugin applying or using the configuration. The means of what constitutes a valid configuration is plugin specific and determined by the plugin internals.

RPC "Validate" will initially take the same input parameters as RPC "Configure", but will be kept in a different type to support future deviations.

RPC "Validate" will return a boolean indicating if the plugin deems the configuration valid, and a user directed string "message" holding text about the passing or failing nature of the validation.

[edwbuck]

Wrong repository, meant for spire-plugin-sdk.

Closing as not planned.