Add usage API support (#14)

Signed-off-by: Milos Gajdos <milosthegajdos@gmail.com>

examples/usage/main.go

@@ -0,0 +1,18 @@
+package main
+
+import (
+	"context"
+	"log"
+
+	"github.com/milosgajdos/go-vocode"
+)
+
+func main() {
+	client := vocode.NewClient()
+
+	usage, err := client.GetUsage(context.Background())
+	if err != nil {
+		log.Fatalf("failed getting usage: %v", err)
+	}
+	log.Printf("got usage: %+v", usage)
+}

usage.go

@@ -0,0 +1,55 @@
+package vocode
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/url"
+
+	"github.com/milosgajdos/go-vocode/request"
+)
+
+type PlanType string
+
+const (
+	PlanFree       PlanType = "plan_free"
+	PlanDeveloper  PlanType = "plan_developer"
+	PlanEnterprise PlanType = "plan_enterprise"
+	PlanUnlimited  PlanType = "plan_unlimited"
+)
+
+type Usage struct {
+	UserID              string   `json:"user_id"`
+	PlanType            PlanType `json:"plan_type"`
+	MonthlyMinutes      int      `json:"monthly_usage_minutes"`
+	MonthlyLimitMinutes int      `json:"monthly_usage_limit_minutes"`
+}
+
+func (c *Client) GetUsage(ctx context.Context) (*Usage, error) {
+	u, err := url.Parse(c.opts.BaseURL + "/" + c.opts.Version + "/usage")
+	if err != nil {
+		return nil, err
+	}
+
+	options := []request.HTTPOption{
+		request.WithBearer(c.opts.APIKey),
+	}
+
+	req, err := request.NewHTTP(ctx, http.MethodGet, u.String(), nil, options...)
+	if err != nil {
+		return nil, err
+	}
+
+	resp, err := request.Do[*APIError](c.opts.HTTPClient, req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+
+	usage := new(Usage)
+	if err := json.NewDecoder(resp.Body).Decode(usage); err != nil {
+		return nil, err
+	}
+
+	return usage, nil
+}