azurerm_api_management_api: split create and update method

[wuxu92]

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Description

Split Update method from CreateUpdate method, and an update test for it.

PR Checklist

• I have followed the guidelines in our Contributing Documentation.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have checked if my changes close any open issues. If so please include appropriate closing keywords below.
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  For example: “resource_name_here - description of change e.g. adding property new_property_name_here”

Testing

• My submission includes Test coverage as described in the Contribution Guide and the tests pass. (if this is not possible for any reason, please include details of why you did or could not add test coverage)

--- PASS: TestAccApiManagementApi_completeUpdate (252.61s)
PASS