Add draft spec for plaintext key exchange protocol

[yusefnapora]

This adds a spec for exchanging keys and peer ids when setting up the plaintext "security" protocol, as discussed in #184.

This sends both peer ids and keys as @raulk outlined - we could potentially just send the keys as in @Warchant's c++ implementation and derive the ids.

Also, I bumped the version in the protocol id, since it seems like sending the key message over will break anyone using the existing /plaintext/1.0.0 protocol id.

[raulk]

LGTM; few minor comments.

[Kubuxu]

I would be for adding a big disclaimer that this spec is only for development purposes and should never be shipped in actual application or library.

[Warchant]

Hm. We have a little bit different encoding of keys on the wire.

We took it from js-libp2p I guess.

Seems rust-libp2p also implements key encoding according to spec, so we're gonna fix this soon.

[Stebalien]

I kind of want to make both sides send their private keys just to make it clear that this should never be enabled in production but that's probably asking for trouble.

[mhchia]

Looks pretty complete. Thanks for the work on specifying the protocol. 🙏

[yusefnapora]

Awesome, thanks for all the reviews! I added everyone in the thread to the IG - please PR or ping me if you'd rather not be. I also added another warning about not using in production - hopefully people get the message.