Support customized header for invoker

[Halomaple]

In many cases we need passing some params like X-UserId from web to backend in HTTP header, but current implementation of js-sdk is blocking the header. See src/implementation/Client/HTTPClient/invoker.ts line 31-36:

    if (method !== HttpMethod.GET) {
      // @ts-ignore
      fetchOptions.headers = {
        'Content-Type': 'application/json'
      };
    }

So I wonder whether we can add new param named header in function invoke to support this? Like async invoke(appId: string, methodName: string, method: HttpMethod = HttpMethod.GET, data: object = {}, header = {}): Promise<object> {}

[shubham1172]

I support this request. It is something already supported by dotnet-sdk https://github.com/dapr/dotnet-sdk/issues/396

[pnocera]

I did a fork and implemented it. That is 6 lines of code....could submit a PR eventually.

[shubham1172]

@pnocera that's great, feel free to raise a PR. Thank you for helping out!