Errors from `volume create` or `volume delete` only get logged by the client
agent, which may make it harder for volume authors to debug these tasks if they
are not also the cluster administrator with access to host logs.
Allow plugins to include an optional error message in their response. Because we
can't count on receiving this response (the error could come before the plugin
executes), we parse this message optimistically and include it only if
available.
Ref: https://hashicorp.atlassian.net/browse/NET-12087
This changeset implements the HTTP API endpoints for Dynamic Host Volumes.
The `GET /v1/volumes` endpoint is shared between CSI and DHV with a query
parameter for the type. In the interest of getting some working handlers
available for use in development (and minimizing the size of the diff to
review), this changeset doesn't do any sort of refactoring of how the existing
List Volumes CSI endpoint works. That will come in a later PR, as will the
corresponding `api` package updates we need to support the CLI.
Ref: https://hashicorp.atlassian.net/browse/NET-11549
