allow to construct mock sender for unit testing

[aslom]

Add NewSenderFromSyncProducer to allow running unit tests with mock sender form outside SDK

[slinkydeveloper]

can you rebase on master?

[aslom]

Following https://github.com/cloudevents/sdk-go/blob/master/pr_guidelines.md#submitting-and-updating-your-pull-request

I have ran

git fetch upstream
git rebase upstream/master
git push -f origin mock_tests_support
git rebase -i upstream/master
git push -f origin mock_tests_support

[slinkydeveloper]

Thank you for the contribution!