Describe the bug

It currently seems to be impossible to add a new public key to user's account using GHMyself. POST /user/keys is the relevant GitHub API endpoint.

To Reproduce
Read docs on GHMyself and observe the method not being present

Expected behavior
`GHMyself::addK

The CloudEventBuilder should fail with the proper exception (look at CloudEventRWException#newInvalidExtensionName) if the user is trying to add an extension with a bad name (as specified here https://github.com/cloudevents/spec/blob/master/spec.md#attribute-naming-convention)

1- hasOnlyOneElementSatisfying is deprecated, we need to use singleElement()

warning: [deprecation] hasOnlyOneElementSatisfying(Consumer<? super ELEMENT>) in AbstractIterableAssert has been deprecated
            .hasOnlyOneElementSatisfying(throwable -> {
            ^

For example:

assertThat(errorMessages)
            .hasSize(1)
            .hasOnlyOneElementSat