Add support for Page's Icon retrieval and updates (#21)

* Add support for Page's Icon retrieval and updates

* Add support for creating a page with an icon

* Fix tests, add icon validation

* Remove unknown icon type `file`

* Add icon support for "create database" endpoint

Co-authored-by: David Stotijn <dstotijn@gmail.com>

icon.go

@@ -0,0 +1,38 @@
+package notion
+
+import "errors"
+
+type IconType string
+
+const (
+	IconTypeEmoji    IconType = "emoji"
+	IconTypeExternal IconType = "external"
+)
+
+// Icon has one non-nil Emoji or External field, denoted by the corresponding
+// IconType.
+type Icon struct {
+	Type IconType `json:"type"`
+
+	Emoji    *string       `json:"emoji,omitempty"`
+	External *IconExternal `json:"external,omitempty"`
+}
+
+type IconExternal struct {
+	URL string `json:"url"`
+}
+
+func (icon Icon) Validate() error {
+	if icon.Type == "" {
+		return errors.New("icon type cannot be empty")
+	}
+
+	if icon.Type == IconTypeEmoji && icon.Emoji == nil {
+		return errors.New("icon emoji cannot be empty")
+	}
+	if icon.Type == IconTypeExternal && icon.External == nil {
+		return errors.New("icon external cannot be empty")
+	}
+
+	return nil
+}