Allow callers of Invoke-GHRestMethod[MultipleResult] to specify additional headers

[HowardWolosky]

Description

When experimenting with the the notifications API, it was necessary to be able to specify an additional header (If-Modified-Since).

Making this support generic by allowing callers to specify any number of additional headers to be included with the REST request.

Issues Fixed

n/a

References

Notifications API (although this doesn't actually implement anything from there)

Checklist

• [x] You actually ran the code that you just wrote, especially if you did just "one last quick change".
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• [x] Static analysis is reporting back clean.
• [x] New/changed code adheres to our coding guidelines.
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• [x] New/changed code continues to support the pipeline.
• [x] Changes to the manifest file follow the manifest guidance.
• [x] Unit tests were added/updated and are all passing. See testing guidelines. This includes making sure that all pipeline input variations have been covered.
• [x] Relevant usage examples have been added/updated in USAGE.md.
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