Not possible to set initial value of RichTextInputBlockElement due to wrong type

[matcornic]

Hello maintainers,

First, thanks a lot for this lib, it's very useful for us.
I encountered a bug: I can't set an initial value for the RichTextInputBlockElement due the type being a string and not a Rich text object

What happened

I tried to use this code, which is compiling on :

	elem := slack.NewRichTextInputBlockElement(nil, "test")
	elem.InitialValue = "test"
	elemInput := slack.NewInputBlock("", slack.NewTextBlockObject("plain_text", "Test:", false, false), nil, elemInput)

But got an invalid_block error on Slack while checking the generated json on the Block kit builder

https://app.slack.com/block-kit-builder/T02F6HSJU2H#%7B%22blocks%22:%5B%7B%22type%22:%22header%22,%22text%22:%7B%22type%22:%22plain_text%22,%22text%22:%22Test%22%7D%7D,%7B%22type%22:%22input%22,%22label%22:%7B%22type%22:%22plain_text%22,%22text%22:%22RulTestes:%22%7D,%22element%22:%7B%22type%22:%22rich_text_input%22,%22action_id%22:%22test%22,%22initial_value%22:%22test%22%7D%7D%5D%7D

Seeing the documentation, it expects a Rich text object https://api.slack.com/reference/block-kit/block-elements#rich_text_input

Versions

• Go: 1.20
• slack-go/slack: v0.12.5