auditing: standard customized Slack message for workflows

[scarlettperry]

Description
Currently, if users want to add more details to the standard Slack audit message, they create the freeform message using metadata available in the API request/response. We should experiment with providing a standard customized Slack audit message utilizing a workflow's protos.

Related: #1519

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

Wireframes, Mockups, or Screenshots (if applicable)