fix: new notion relation requires new properties

[brunoluiz]

If you try to use this package as it is together with relations it will not work. This because Notion introduced two new fields that seem to have been a breaking change (although they say it was not). The changelog is available here.

This pull request adds the new fields and add the required constants to be used. A relation then needs to be used as:

			"Something": notionapi.RelationPropertyConfig{
				Type: notionapi.PropertyConfigTypeRelation,
				Relation: notionapi.RelationConfig{
					DatabaseID:     tables.Venues,
					Type:           notionapi.RelationSingleProperty, // NEW
					SingleProperty: &notionapi.SingleProperty{}, // NEW
				},
			},

[jomei]

Thanks a lot!