Add support for Rich Text Input

[peimanja]

closes #1239

[floydjones1]

@kanata2 @parsley42 could we get someone to take a look at this please? 🤞
I can help if there is anything I can do to speed this up

[parsley42]

Hi @floydjones1 - I haven't heard from @kanata2 in some time; he did a great job maintaining for a while, but most likely he's busy with other things now. I do occasional reviews and merges, but I'm more a DevOps engineer than Go dev - honestly, I'm kind of a lousy Go dev, but I've been getting new maintainers on board for a few years now. Unfortunately volunteer maintainers have really dried up - we get a few PRs, but so far nobody has taken on the role. If you or somebody you know is an experienced Go dev and would like to volunteer as a maintainer, I'd be happy to onboard somebody new.

[parsley42]

Looks harmless.

[chase-allen-tech]

Seems like this integration needs additional API modification to read rich text in the request.
Here blockkit shows Text to be slice of object, but TextBlockObject only require string.
I can't get request value of the rick text.
peimanja could you have a look?
CC: parsley42