Make HTTPS requests appear as HTTPS for HTTP triggers on Linux dedicated/premium/(consumption?)

[brettsam]

Discussion for #1926

[brettsam]

@ConnorMcMahon -- you mentioned that you've provided customers with an env var in the past that enabled this for them. Can you add that here?

[brettsam]

https://devblogs.microsoft.com/aspnet/forwarded-headers-middleware-updates-in-net-core-3-0-preview-6/

[brettsam]

Notes

• Rather than host.json, any back-compat support would be using the built-in feature flags.
• Treat this as a bug fix, conditional on linux.
• Likely no need for a feature flag. If needed, we can roll out out after-the-fact
• Cannot think about a reliable way to detect if anyone is using this, so no detector work.

[fabiocav]

For this change, no back-compat/feature flag required.