Dependencies update + ExAws.Operation tests

[stjhimy]

• Exclude "s3 external" tests by default since it requires having a key/secret and an active internet connection to run. Those tests are still runnable using mix test --include external.
• Add ExAws.Operation tests: This will only test the ExAws.Operation building process, checking params etc It can run without internet connection.
• Add a basic usage example on the README so new users can see at first glance how easy is to use it.

Let me know if the operation tests are useful and I can complete the testing before merging.
Thanks a lot for the package, it's been extensively used and very helpful.

[andreragsilva]

@stjhimy thank you for the contribution 🙏

I am quite busy at the moment, so I will be reviewing the PR on Friday 🤞

[andreragsilva]

Thank you for the help @stjhimy, it definitely improves the testing structure 🙌

I did start something similar within branch feature/add-missing-actions but I didn't finish since the branch contains issue #13.

[andreragsilva]

To answer your question, the tests are useful and there will be more Operation Actions that will require testing as well. Please check the branch I mentioned above and feel free to contribute.

Sorry for the delay, have been a couple busy weeks 👍

[stjhimy]

Great @andreragsilva!
I will have a look at the branch this week and see if I can move on with #13 💪