Slack plugin enhancements

[jongpie]

• Move package to be in the same dev hub as the main logger packages. Currently, it's in a separate dev hub, which makes things harder to manage.
• Incorporate LogEntryTag__c and LoggerTag__c objects into the plugin. Currently, it only uses Topic and TopicAssignment
• Add field set to control the Slack message's contents. Currently, the list of fields used in the Slack plugin are hardcoded within the Apex class - it'd be nice if this was instead controlled via a field set on LogEntry__c so orgs can customize their Slack messages
• Add the ability to further filter which LogEntry__c records are posted to Slack - currently, only the logging level can be used, but it would be useful to be able to also filter/exclude records based on wildcard/regex matches in LogEntry__c.Message__c