Adding exclude_fields in the configuration of slack handlers

[Gwemox]

Add configuration :
exclude_fields => Dot separated list of fields to exclude from slack message. E.g. ['context.field1', 'extra.field2']

cf. https://github.com/Seldaek/monolog/blob/main/src/Monolog/Handler/SlackHandler.php

[doenietzomoeilijk]

I would love to see this land in a release, I'm currently running into a situation where this functionality is pretty much required. Can't solve it with a processor, since the record is already turned into a SlackRecord...

[doenietzomoeilijk]

So... any way of getting this ball rolling again? Pinging @Seldaek / @stof ?

[Gwemox]

Hi @markosea,
I've just rebased my branch with master and resolved the conflicts. Could you please review it again when you have a moment?
Thanks!