Add sharing info to /workspaces endpoint

[aslilac]

type Workspace struct {
	// unchanged fields omitted
	SharedWith []SharedWorkspaceActor `json:"shared_with"`
}

type SharedWorkspaceActor struct {
	// Definitely useful
	ActorType SharedWorkspaceActorType `json:"actor_type" enums:"group,user"`
	Name      string    `json:"name"`
	AvatarURL string    `json:"avatar_url,omitempty" format:"uri"`
	Roles []WorkspaceRole `json:"roles"`

	// more fields might be added if necessary, but this is minimal and useful
}

type SharedWorkspaceActorType string

const (
	SharedWorkspaceActorTypeGroup SharedWorkspaceActorType = "group"
	SharedWorkspaceActorTypeUser  SharedWorkspaceActorType = "user"
)

[aslilac]

Bumping complexity a bit because we probably need to make decisions here about if showing who a workspace is shared with is sensitive information. We probably need to run the actors you can see through the usual dbauthz path and then either hide them entirely or show a placeholder "you're not allowed to know who this is" actor.