PoC for a masque proxy

[pinkisemils]

This is a small PoC implementation of a masque. We could use a masque proxy almost as is, if it wasn't for the exorbitant header sizes which interfere with multihop. For the larger packet sizes for multihop, which then enables DAITA, we are looking to extend the protocol to support fragmented packets.

We are looking to use masque to obfuscate our traffic to make it appear as though it was regular QUIC traffic. I'm laying this here so that we have a clear path towards implementing a QUIC Masque proxy for ourselves in the upcoming quarter - we don't need to merge this code as is.

In terms of libraries, I've had to extend h3 to make use of it, namely, exposing QUIC datagrams to client connections. Other than that, we should be able to use off-the-shelf libraries. For TLS, this is using the exact same rustls as we're using in mullvad-api. We could move the whole implementation into the obfuscation crate for cleanliness.

I have tested this with local iperf2, the performance isn't that great, 500mbit/s, yet to test the impact on WireGuard traffic

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This change is 

[pinkisemils]

Whilst the crate here depends on a fork of ours, we can use upstream h3,as the changes we needed have been merged. They just have not made a release with them yet.