feat: add Python send-message plugin

[snickell]

Python implementation of PR #5; adds send-message for Python.

Main PR:

• PR feat: add send-message step plugin base and Go implementation #5: feat: add send-message step plugin base and Go implementation #5

Proposal links:

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

Technical changes:

• add the Python plugin under extended/plugins/send-message-python
• add Python-specific proposal 0004 plan and checklist documents
• add a Python-specific send-message smoke path
• carry the shared harness changes needed to run the Python smoke path end to end while this PR is reviewed separately