Add blocks for messages.

[nliutyi-hs]

This PR adds message blocks, which are passed by Slack in the message_action payload. This will allow us to work with user messages in a much more sufficient way, removing the need to parse the Slack markdown text itself, but giving the flexibility to work with blocks.

We've used this code version in our QA env and there were no exceptions regarding to it in our runtime.

[nliutyi-hs]

@motnyk Regarding the example payload, I thought that message_action.json is a good example, and there is no need to duplicate it, but I should've mentioned this in the PR's description.

[nliutyi-hs]

@motnyk Gonna simplify the payload and build the message blocks in the code, for decent comparison.

[motnyk]

Regarding the example payload, I thought that message_action.json is a good example, and there is no need to duplicate it, but I should've mentioned this in the PR's description.

That's a good call, I haven't really gotten to it yet at a point when I realized that I missed an example to simplify the review.

[nliutyi-hs]

@motnyk PR is updated, please take a look once you'll be able to.