[TCGC] Add External usage flag for types referenced by external types

[Copilot]

• Add External flag to UsageFlags enum in interfaces.ts
• Add logic to propagate External usage recursively for types referenced by external types
• Add tests to verify External usage flag behavior including transitive references
• Add changelog entry
• Use updateUsageOrAccess for External flag propagation
• Consolidate External usage propagation into single function called after spread usage update
• Update tests to expect External flag on external types themselves
• Format code with prettier
• Verify tests pass

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[tadelesh]

@copilot do not update external usage everywhere, do it after update spread usage.

[Copilot]

@copilot do not update external usage everywhere, do it after update spread usage.

Consolidated all External usage updates into a single updateExternalUsage() function that's called in handleAllTypes right after updateSpreadModelUsageAndAccess(). Removed the 8 scattered checks from individual type creation functions. (commit 7d8e2a5)

[tadelesh]

@copilot

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/Azure/typespec-azure/@azure-tools/typespec-client-generator-core@3422

commit: 4fdfeb5

[tadelesh]

@copilot format the code

[Copilot]

@copilot format the code

Code formatted with prettier. (commit ab3c9d1)

[msyyc]

Need more discussion.
Update: current design may meet issue for scenario that shared model across external type and normal type. But That scenario may not appear so let us go on.