[Slack] arrange Slack APIs into native structure

[darkbtf]

It would be better to support APIs in Slacks' native API structures

e.g.
conversations.info

https://github.com/Yoctol/messaging-apis/blob/b83f9186f7e2f477caf29ffec675a24d0d518c68/packages/messaging-api-slack/src/SlackOAuthClient.ts#L177-L190

current API:
SlackOAuthClient.getConversationInfo

Slack's native structure:
SlackOAuthClient.conversations.info