fix codegen when a model explicitly specifies the enum value NOT_SET

[sbiscigl]

Description of changes:

If a service model uses the enum value NOT_SET it will conflict with the SDKs implicit enum value of NOT_SET which is encountered when:

• an enum cannot be deserialized from a string value
• accessing an enum created from an invalid string value

This re-maps the modeled value from NOT_SET to NOT_SET_VALUE. this really should be modeled as a optional in a future breaking release.

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

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