Expose the Account API (RADIUS users)

* Adds Account CRUD API
* Enahnces `emptyStringInt` with a predicate for dynamically setting
  which value should be interpreted as a blank JSON payload

unifi/account.go

@@ -0,0 +1,71 @@
+package unifi
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+)
+
+func (dst *Account) UnmarshalJSON(b []byte) error {
+	type Alias Account
+	aux := &struct {
+		TunnelType       emptyStringInt `json:"tunnel_type"`
+		TunnelMediumType emptyStringInt `json:"tunnel_medium_type"`
+		VLAN             emptyStringInt `json:"vlan"`
+
+		*Alias
+	}{
+		Alias: (*Alias)(dst),
+	}
+
+	err := json.Unmarshal(b, &aux)
+	if err != nil {
+		return fmt.Errorf("unable to unmarshal alias: %w", err)
+	}
+
+	dst.TunnelType = int(aux.TunnelType)
+	dst.TunnelMediumType = int(aux.TunnelMediumType)
+	dst.VLAN = int(aux.VLAN)
+
+	return nil
+}
+
+func (dst *Account) MarshalJSON() ([]byte, error) {
+	type Alias Account
+	aux := &struct {
+		*Alias
+
+		TunnelType       emptyStringInt `json:"tunnel_type"`
+		TunnelMediumType emptyStringInt `json:"tunnel_medium_type"`
+		VLAN             emptyStringInt `json:"vlan"`
+	}{
+		Alias: (*Alias)(dst),
+	}
+
+	aux.TunnelType = emptyStringInt(dst.TunnelType)
+	aux.TunnelMediumType = emptyStringInt(dst.TunnelMediumType)
+	aux.VLAN = emptyStringInt(dst.VLAN)
+
+	b, err := json.Marshal(aux)
+	return b, err
+}
+
+func (c *Client) ListAccount(ctx context.Context, site string) ([]Account, error) {
+	return c.listAccount(ctx, site)
+}
+
+func (c *Client) GetAccount(ctx context.Context, site, id string) (*Account, error) {
+	return c.getAccount(ctx, site, id)
+}
+
+func (c *Client) DeleteAccount(ctx context.Context, site, id string) error {
+	return c.deleteAccount(ctx, site, id)
+}
+
+func (c *Client) CreateAccount(ctx context.Context, site string, d *Account) (*Account, error) {
+	return c.createAccount(ctx, site, d)
+}
+
+func (c *Client) UpdateAccount(ctx context.Context, site string, d *Account) (*Account, error) {
+	return c.updateAccount(ctx, site, d)
+}