Add support for user attribute "nickname"

[tkeber]

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Changes proposed in this pull request:

• Add support for user attribute nickname.
• Fix testAccUserCreateUser (username is required property)

Output from acceptance testing:

➜ make testacc TESTS=TestAccUserCreateUser
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./...) -v  -timeout 120m -coverprofile="c.out" -run ^TestAccUserCreateUser
?       github.com/alexkappa/terraform-provider-auth0   [no test files]
=== RUN   TestAccUserCreateUser
--- PASS: TestAccUserCreateUser (0.68s)
PASS
coverage: 11.8% of statements
ok      github.com/alexkappa/terraform-provider-auth0/auth0     0.701s  coverage: 11.8% of statements


...

[lstiebel]

Hey, just to follow up on this (@tkeber is a colleague of mine), is there anything we can do to assist in getting this merged?

Would allow us to switch back from a fork to the upstream repository.

[alexkappa]

@tkeber, @lstiebel, I've rebased and merged this offline. Thank you for submitting this! I am not sure why the build was failing, everything seemed ok to me 🤷‍♂