Filter props for <ConversationsSelect> (experimental)

[yhatt]

Slack has shipped filter field for conversations select and its multiple version as beta.
https://api.slack.com/reference/block-kit/composition-objects#filter_conversations

If it was implemented in jsx-slack's <ConversationsSelect> component, I suppose that would become to specify in properties directly instead of using composition object.

<Blocks>
  <Section>
    Select DM channel:
    <ConversationsSelect
      multiple
      include={['im', 'mpim']}
      excludeBotUsers
    />
  </Section>
</Blocks>

During beta, jsx-slack should mark these props as experimental. There are known issues in iOS.