docs(extensibility): name extension base capabilities

[ericcurtin]

Summary

Name the base capability each extension family must declare.

Related Issue

Closes #4378

Changes

• Docs: table of base capabilities with an example manifest.
• Proto: name the expected values next to each extension field.
• Missing metadata and missing version errors now name the expected protocol and capability.

Testing

• Checks appropriate to the affected code and behavior pass
• Unit tests added/updated (if applicable)
• E2E tests added/updated (if applicable)

cargo fmt, cargo clippy and cargo test for openshell-core. mise run docs passes.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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[ericcurtin]

@johntmyers @elezar PTAL when you get a chance, and /ok to test 0bf47e0f89e757c31837d4a1ced86fb4c10f90eb if it looks good. Thank you!