No filter object in multi_conversation_select

[imamfzn]

Hi team, currently there is no filter field in the multi_conversation_select block element type like below:

slack/block_element.go

Lines 270 to 283 in b4b5a64

	type MultiSelectBlockElement struct {
	Type string `json:"type,omitempty"`
	Placeholder *TextBlockObject `json:"placeholder,omitempty"`
	ActionID string `json:"action_id,omitempty"`
	Options []*OptionBlockObject `json:"options,omitempty"`
	OptionGroups []*OptionGroupBlockObject `json:"option_groups,omitempty"`
	InitialOptions []*OptionBlockObject `json:"initial_options,omitempty"`
	InitialUsers []string `json:"initial_users,omitempty"`
	InitialConversations []string `json:"initial_conversations,omitempty"`
	InitialChannels []string `json:"initial_channels,omitempty"`
	MinQueryLength *int `json:"min_query_length,omitempty"`
	MaxSelectedItems *int `json:"max_selected_items,omitempty"`
	Confirm *ConfirmationBlockObject `json:"confirm,omitempty"`
	}

At the moment, only the conversation_select which has the filter object. In the slack documentation, it said the filter object also supported in the multi_conversation_select.

• https://api.slack.com/reference/block-kit/block-elements#conversation_multi_select
• https://api.slack.com/reference/block-kit/composition-objects#filter_conversations

As a workaround to bypass this issue, i create a conversation_select block element, and change the element type once it created to be multi_conversation_select, like:

var modal slack.ModalViewRequest
// omitted code to parsed/unmarshal the json kit to be the the view struct
// ...

block := modal.Blocks.BlockSet[0].(*slack.SectionBlock)
block.Accessory.SelectElement.Type = "multi_conversations_select"
modal.Blocks.BlockSet[0] = block

Example json kit template used

{
  "type": "modal",
  "title": {
    "type": "plain_text",
    "text": "Add to Channel",
    "emoji": true
  },
  "submit": {
    "type": "plain_text",
    "text": "Add",
    "emoji": true
  },
  "close": {
    "type": "plain_text",
    "text": "Close",
    "emoji": true
  },
  "blocks": [
    {
      "type": "section",
      "block_id": "block_add_to_channel",
      "text": {
        "type": "mrkdwn",
        "text": "*Add these people to {{.ChannelName}}*"
      },
      "accessory": {
        "action_id": "action_add_to_channel",
        "type": "conversations_select",
        "placeholder": {
          "type": "plain_text",
          "text": "Add people and/or channels"
        },
        "filter": {
          "include": [
            "im",
            "public"
          ],
          "exclude_bot_users": true
        }
      }
    }
  ]
}

But i think that way is not clean, since we actually can directly create the multi_conversation_select with filter object based on the slack documentation.

I create my own pull request in #1191, please help to review and merge / please help to fix that issue if my PR is not suitable.

Thank you!