Add a validateConfig() API

[mandarjog]

A wasm extension implements onConfigure(std::unique_ptr<WasmData> configuration)
If configuration fails, there is no indication that something has gone wrong except a WARN message from the extension itself.

1. onConfigure() should return a bool
2. Add a validateConfig(std::unique_ptr<WasmData> configuration) API which is similar to onConfigure
   • It must do structural validation of the input configuration.
   • It must be stateless, must be a pure function and must not use the HOST api.

A WASM extension that supports validateConfig can be used outside the proxy as a config validator.

[mandarjog]

@kyessenov @douglas-reid

[kyessenov]

What happens if config is invalid? Should we reject LDS?