Added API versioning support

[variableresistor]

Description

GitHub is introducing versioned API's. In order to get the expected behavior of an API, you need to invoke the API with its version in the request header. This change updates the core functions (Invoke-GHRestMethod and Invoke-GHRestMethodMultipleResult) to accept an ApiVersion as part of their param sets so that as the module continues to add new functionality, methods can request the appropriate API Version for their individual calls.

Features Added

Fixes #377

References

To infinity and beyond: enabling the future of GitHub’s REST API with API versioning
API Versions

Checklist

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[variableresistor]

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[HowardWolosky]

Thanks for your contribution, @variableresistor! I also appreciate you calling attention to the fact that GitHub is introducing API versioning...I completely missed that announcement.

I love the idea of making the module API version aware, but I currently disagree with the methodology that you're using to introduce it. We're likely to adopt newer API versions on a per-API basis, one at a time. That means we wouldn't want a module-wide mechanism for changing the API Version, but rather the ability to change it on every REST call.

[HowardWolosky]

I thought I had published this review earlier, but it still shows as pending.

[HowardWolosky]

Super minor nits, and then should be good to merge.

[HowardWolosky]

/azp run PowerShellForGitHub-CI

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[HowardWolosky]

Looks good now. Thanks for the contribution and for noticing this pending change in the first place!

Tests are sufficiently passing (pretty much all of the failing tests are the Projects tests which we know will fail (due to #380) -- they're about to be disabled).