Add to slack source ability to exclude channels

[VioletM]

Which source is this for
Slack

Describe the data you'd like, be specific, link the docs
Currently, there is no way to specify which channels have to be excluded. I suggest adding two parameters:

1. exclude_archived: bool -- whether to exclude archived parameters
2. exclude_channels: List[str] -- list of channels to exclude (add all channels, which are not in exclude list)