ALPN and WebRTC configuration

[brpettis]

Expose the ability to enable or disable alpn header for WebRTC traffic.

Being a fairly new label via the RFC: https://www.rfc-editor.org/rfc/rfc8833 if proxies don't allow or fail to parse the label's correctly it will allow a workaround preventing users from having to disable ALPN all together.

Something like:
media.security.enable_alpn

[Jon]

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