auditing: move customized Slack messages out of the clutch-config

[scarlettperry]

Description
Currently, users enable/create customized slack message in the clutch-config. We should move these custom messages alongside a workflow's API code so that there's less configuration in the clutch-config.

Relevant files to look at:

• https://github.com/lyft/clutch/blob/main/api/audit/v1/audit.proto
• https://github.com/lyft/clutch/blob/main/api/config/service/auditsink/slack/v1/slack.proto
• https://github.com/lyft/clutch/blob/main/backend/service/auditsink/slack/slack_helper.go
• https://github.com/lyft/clutch/blob/main/backend/service/auditsink/slack/slack.go

Complexity [S/M/L]: M