feat: fetchOptions, callOptions for React Native

[wackyleo459]

Description

For React Native project, to create new fetch settings that work with React-Native-Fetch-API polyfill.

new HttpAgent ({
    fetchOptions: {
      reactNative: {
        __nativeResponseType: "base64",
      },
    },
    callOptions: {
      reactNative: {
        textStreaming: true,
      },
    },
    host: "http://localhost:8000",
})

When source isn't provided in constructing the HttpAgent, this._fetchOptions and this._callOptions will automatically pull from provided options. Currently this is the only place where fetchOptions and callOptions will be set.

this._fetchOptions = options.fetchOptions; this._callOptions = options.callOptions;