Force new resource when auth0_rule_config.key has change

[alexkappa]

Proposed Changes

• Set ForceNew: true for auth0_rule_config.key.
• Add more tests to verify the correct behavior.

Fixes #214

Acceptance Test Output

$ make testacc TESTS=TestAccRuleConfig                   [14:21:01]
==> Checking that code complies with gofmt requirements...
?       github.com/terraform-providers/terraform-provider-auth0 [no test files]
=== RUN   TestAccRuleConfig
--- PASS: TestAccRuleConfig (2.28s)
PASS
coverage: 7.2% of statements
ok      github.com/terraform-providers/terraform-provider-auth0/auth0   2.596s  coverage: 7.2% of statements

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