The function proxy_get_configuration() should be removed from the SDK

[mkauf]

The function proxy_get_configuration() is not implemented anymore in Envoy. If it's used by a WASM plugin, Envoy prints this error message:

[error][wasm] [source/extensions/common/wasm/wasm_vm.cc:38] Failed to load Wasm module due to a missing import: env.proxy_get_configuration

Therefore the function proxy_get_configuration() should be removed from the SDK.

Note: To get the plugin configuration, use getBufferBytes(WasmBufferType::PluginConfiguration, 0, configuration_size).