Support expand Deployment Statuses API

[unblee]

Fixes #1035

Changes:

• Add custom media type(application/vnd.github.flash-preview+json)
• Add environment parameter to DeploymentStatusRequest struct

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[unblee]

I signed it!

[googlebot]

CLAs look good, thanks!

[gmlewis]

Thank you, @unblee!
LGTM.
Awaiting second LGTM before merging.

[gauntface]

LGTM from me too and thank you for the discussion in the original issue to claim this work before getting started 🥇

[gmlewis]

Thank you, @gauntface.
Merging.

[unblee]

Thank you, @urohit011 @gmlewis @gauntface 😄