Fix DatabasePropertyFilter.select

[rock88]

Hello! According to Notion’s documentation, the select filter does not support the contains option. Instead, the equals option should be used:

I added SelectCondition to support this, but it's the same as StatusCondition, so feel free to suggest your ideas on how to reduce code duplication. For example, using a typealias could be an option:

extension DatabasePropertyFilter {
    public typealias SelectCondition = StatusCondition
}

Or extract it into a new generic condition.