Add rich_text classes to slack_sdk.models module about python-slack-sdk

[seratch]

Hi @tj-smith47, thanks for writing in!

'm wondering if there is a plan to add a model for this in place, and if so, where that is on the priority scale?

Since Slack's platform unlocked those rich_text blocks to 3rd party apps recently, this project has not yet supported it. However, we definitely plan to add it! (Actually it's already done for Node and Java SDKs).

I cannot tell the exact date to publish a new version yet, but it won't take weeks for sure. If everything goes well, a new version including it would be available by the end of this week.

Also hoping to get clarity on what appears should be LinkTextObject for use inside a section or list of a RichTextElement. Passing subtype="link" to TextObject "works", but the url property gets discarded.

I won't reuse the TextObject class for the link item in rich text list objects. The TextObject represents a typical property of a block, which possesses either a "plain_text" or "mrkdwn" type (and usually it is supposed to be used for label etc.). This object cannot be an item of "rich_text_section" block element. They are completely different ones. So, when we add rich_text block elements to the model module, we will introduce a new class for the link item.

[tj-smith47]

@seratch This is great news! Thank you for your work on this, can't wait to see it!