Convert Slack plugin to use the new Apex Slack SDK

[jongpie]

The new Apex SDK for Slack is currently in beta - I'd like to consider migrating the existing Slack plugin to the new native Apex SDK, instead of the current approach of using custom callouts and DTOs for integrating.

I've done a very basic prototype already, but more analysis needs to be done to see what's feasible in the Apex SDK. In particular, I'd like to figure out if:

• The Slack app can be used to send logs from multiple Salesforce environments to a single Slack workspace/channel
• The Slack app can be used to send specific logs/log entries (based on some criteria/criterion) to different Slack workspaces/channels. This is not supported in the current plugin, and would be great for orgs where different teams are responsible for different logs. LoggerScenario__mdt could be one option for configuring what Slack channel is used when sending notifications.
• The Slack app can still support using webhooks. Webhooks are supported in the current version of Nebula Logger's Slack plugin, so I'd like to keep this as a feature.
• The Slack app supports displaying & updating Salesforce log & log entry records directly within Slack - the current plugin displays some fields in the initial message, but to see more details or edit the record, you have to go into the corresponding Salesforce org

Given that the Apex SDK is still beta, this isn't top priority, but it'd still be great to explore this more.