NIFI-4619: Enable EL on AWSCredentialsProviderControllerService properties

[mgaido91]

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[mgaido91]

@jvwing @mans2singh may I kindly ask you some help reviewing this? I saw you are the last contributors on it... Thanks!

[jvwing]

Reviewing...

[jvwing]

Thanks, @mgaido91! The code change looks good and works good. And thanks for tuning up the tests appropriately.

[mgaido91]

thank you for your review @jvwing!