Enable support in dev tunnels for customizing forwarded headers

Dev tunnels support customizing the value forwarded for both the `Host` and `Origin` headers. It defaults to always setting these headers to "localhost" for forwarded requests but supports opting in to preserving the original values, or even setting a custom value. We should expose this capability in the dev tunnels hosting integration.

Start by locating the dev tunnels hosting integration and reading how it configures forwarded requests. Confirm how Host and Origin values are currently set, then expose the supported customization options and verify that forwarded requests honor the selected behavior.

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