[api-go] Support direct Payload's and Payload`s at top-level of Any in payload visitor

[yuandrew]

Is your feature request related to a problem? Please describe.
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
When creating a common.Payload and marshaling it into an Any type, the visitor is not able to visit this scenario, due to a missing case *common.Payload in visitPayloads().

This scenario is not used in our API today, but something we may want to support in the future.

Describe the solution you'd like
Implement this base case so we can visit this scenario.

Additional context
Here is a basic test I wrote to validate this new feature

func TestVisitPayloads_AnyPayload(t *testing.T) {
	msg, err := anypb.New(inputPayload())
	require.NoError(t, err)
	root := &protocol.Message{Body: msg}
	var anyCount int
	err = VisitPayloads(context.Background(), root, VisitPayloadsOptions{
		Visitor: func(ctx *VisitPayloadsContext, p []*common.Payload) ([]*common.Payload, error) {
			anyCount++
			// Only mutate if the payloads has "test"
			if len(p) == 1 && string(p[0].Data) == "test" {
				return []*common.Payload{{Data: []byte("new-val")}}, nil
			}
			return p, nil
		},
	})
	require.NoError(t, err)
	require.Equal(t, 1, anyCount)
	update1, err := root.Body.UnmarshalNew()
	require.NoError(t, err)
	require.Equal(t, "new-val", string(update1.(*common.Payload).Data))
}

also see temporalio/api-go#202 (comment) for more context

[yuandrew]

When this is implemented, this is another test that can be added

func TestProtocolMessage(t *testing.T) {
	require := require.New(t)

	var messageType protoreflect.MessageType
	protoregistry.GlobalTypes.RangeMessages(func(mt protoreflect.MessageType) bool {
		if string(mt.Descriptor().FullName()) == "temporal.api.protocol.v1.Message" {
			messageType = mt
		}
		return true
	})

	// Create empty instance and populate with test values
	msg := messageType.New().Interface().(proto.Message)
	var totalCount, count int
	populatePayload(&msg, msg, require, &totalCount, &count)

	require.Equal(0, count)
	require.Equal(1, totalCount)
}