Support handlers that return io.Reader

[bmoffatt]

Issue #, if available:

#195

Description of changes:

Without this change, to return arbitrary non-json bytes, a user must write something like this

type handler struct{}

func (h *handler) Invoke(_ context.Context, req []byte) ([]byte, error) {
	var name string
	if err := json.Unmarshal(req, &name); err != nil {
		return nil, err
	}
	return []byte("hello " + name), nil
}

func main() {
	lambda.Start(&handler)
}

This works, but isn't perfect:

1. extra boilerplate to define a type that can implement Invoke
2. handler is forced to implement it's own unmarshalling of the request

With this change, the above handler can be re-written like:

func main() {
	lambda.Start(func(name string) (io.Reader, error) {
		return strings.NewReader("hello " + name), nil
	})
}

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

Merging #472 (a6eae08) into main (0f46684) will increase coverage by 1.49%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##             main     #472      +/-   ##
==========================================
+ Coverage   69.69%   71.18%   +1.49%     
==========================================
  Files          20       21       +1     
  Lines        1201     1253      +52     
==========================================
+ Hits          837      892      +55     
+ Misses        297      295       -2     
+ Partials       67       66       -1     

Impacted Files	Coverage Δ
lambda/entry.go	81.81% <ø> (ø)
lambda/invoke_loop.go	90.17% <83.33%> (+0.75%)	⬆️
lambda/handler.go	95.58% <94.23%> (+3.49%)	⬆️
lambda/runtime_api_client.go	73.13% <100.00%> (ø)
lambda/entry_generic.go	100.00% <0.00%> (ø)

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