[Storage] Error when using `@@alternateType` for SDK crate-defined model

[vincenttran-msft]

.tsp: Azure/azure-rest-api-specs#42277
Full output:

error @azure-tools/typespec-rust/InternalError: The emitter encountered an internal error during preprocessing. Please open an issue at https://github.com/Azure/typespec-rust/issues and include the complete error message.
unhandled headerScalar param type kind external
Error: unhandled headerScalar param type kind external
at getHeaderPathQueryParamValue (file:///C:/azure-sdk-for-rust/sdk/storage/azure_storage_blob/TempTypeSpecFiles/node_modules/@azure-tools/typespec-rust/src/codegen/clients.ts:2043:15)
at Object.body (file:///C:/azure-sdk-for-rust/sdk/storage/azure_storage_blob/TempTypeSpecFiles/node_modules/@azure-tools/typespec-rust/src/codegen/clients.ts:1140:103)
at Module.buildIfBlock (file:///C:/azure-sdk-for-rust/sdk/storage/azure_storage_blob/TempTypeSpecFiles/node_modules/@azure-tools/typespec-rust/src/codegen/helpers.ts:423:19)
at getParamValueHelper (file:///C:/azure-sdk-for-rust/sdk/storage/azure_storage_blob/TempTypeSpecFiles/node_modules/@azure-tools/typespec-rust/src/codegen/clients.ts:884:39)
at applyHeaderParams (file:///C:/azure-sdk-for-rust/sdk/storage/azure_storage_blob/TempTypeSpecFiles/node_modules/@azure-tools/typespec-rust/src/codegen/clients.ts:1132:13)
at constructRequest (file:///C:/azure-sdk-for-rust/sdk/storage/azure_storage_blob/TempTypeSpecFiles/node_modules/@azure-tools/typespec-rust/src/codegen/clients.ts:1173:11)
at getAsyncMethodBody (file:///C:/azure-sdk-for-rust/sdk/storage/azure_storage_blob/TempTypeSpecFiles/node_modules/@azure-tools/typespec-rust/src/codegen/clients.ts:1333:25)
at methodBody (file:///C:/azure-sdk-for-rust/sdk/storage/azure_storage_blob/TempTypeSpecFiles/node_modules/@azure-tools/typespec-rust/src/codegen/clients.ts:217:20)
at emitClients (file:///C:/azure-sdk-for-rust/sdk/storage/azure_storage_blob/TempTypeSpecFiles/node_modules/@azure-tools/typespec-rust/src/codegen/clients.ts:265:40)
at file:///C:/azure-sdk-for-rust/sdk/storage/azure_storage_blob/TempTypeSpecFiles/node_modules/@azure-tools/typespec-rust/src/codegen/codeGenerator.ts:86:29

Failed to generate client

Once we get support for this scenario, we would be able to remove all the into() from HttpRange to coerce it into String for APIs such as range parameter in upload_page.
Azure/azure-sdk-for-rust#4145