Support emoji reactions across runs/restarts

### Your checklist for this issue

- [x] Jenkins version
Jenkins 2.235.5
- [x] Plugin version
Slack Notification Plugin 2.43
- [x] OS
Linux 4.19.112+

### Description

As stated in the Readme, I'll file an issue as I wish to have support for Emoji reactions across restarts and pipeline runs. The latter one is especially useful for me as I trigger a Slack message by `post { failure { } }` and want to mark it resolved by a green check mark emoji with `post { fixed { } }`. This is currently not possible as I can't store the message/thread ID for subsequent runs and have to use the object returned by `slackSend` (which I can not store).

My current workaround is to post an answer to the message/thread ID (which works solely by ID) but it'd be much easier to have an emoji reaction which is shown directly instead of an answer.

Start with the README section describing emoji reactions and the slackSend step behavior. Trace how the message or thread ID returned by slackSend is handled across post failure and fixed blocks, then determine what persistence across pipeline runs and Jenkins restarts would require. Done means a reaction can be applied to the earlier message after a later run.

Written by the indexing model from the issue text.