feat: add send-message step plugin base and Go implementation

[snickell]

Add the Go implementation of the send-message StepPlugin for proposal 0004.

This PR also adds the shared smoke and e2e harness changes needed to run send-message end to end, with Go as the implementation used here.

The follow-on language ports are split into separate PRs:

• PR feat: add Python send-message plugin #6 adds the Python implementation of send-message
• PR feat: add Ruby send-message plugin #7 adds the Ruby implementation of send-message
• PR feat: add TypeScript send-message plugin #8 adds the TypeScript implementation of send-message

Proposal links:

• Proposal directory: https://github.com/code-dot-org/kargo-extended/tree/proposal/0004-send-message-step-plugin/extended/docs/proposals/0004-send-message-step-plugin
• Proposal document: https://github.com/code-dot-org/kargo-extended/blob/proposal/0004-send-message-step-plugin/extended/docs/proposals/0004-send-message-step-plugin/proposal.md

Technical changes:

• add the Go send-message plugin under extended/plugins/send-message-go
• update the shared stepplugin e2e harness to run send-message smoke by implementation
• use Go as the default implementation for this smoke path
• make the Go smoke output identify itself in Slack and in the generated stage name
• keep the existing Go tests and isolated kind smoke path for proposal 0004