Set a User-Agent for API requests

[jfretheim]

Currently, the User-Agent sent to Jamf Pro is Microsoft.Data.Mashup (https://go.microsoft.com/fwlink/?LinkID=304225). For better monitoring and metrics, let's set a specific value that includes the version (something like Jamf Pro Power BI Connector/2.0.0).

A good set of guidelines for User-Agent values can be found here: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/User-Agent

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Considerations/nice-to-haves for the "comment" section of a User-Agent string:

• build metadata and/or branch name for non-release builds
• information about whether request is coming from Power BI Desktop or powerbi.com (if possible)
• the table being populated for the API call
  • down the road, if a cleaner/modern data model is added to this connector, we may want this additional metric to help us decide when it is safe to deprecate or remove the code that supports the old tables (this is a pretty long-term consideration, but worth thinking about as part of this issue)