Jaro Spisak

Add support for Bookmarks API

1

### Description Slack API has added support for bookmarks. It would be great to have that available in the go library. ### (Optional) Slack's documentation https://api.slack.com/apis/bookmarks-api

Add support for title argument in CreateChannelCanvas method

2

This PR adds support for `title` argument in the `CreateChannelCanvas` method. Closes #1482.

### Description [conversations.canvases.create](https://docs.slack.dev/reference/methods/conversations.canvases.create/) method now supports passing `title` argument. Add support for the argument to the `CreateChannelCanvas` method. ### (Optional) Slack's documentation https://docs.slack.dev/reference/methods/conversations.canvases.create/