core: Add Serialize and Deserialize to PeerId and MessageId

[laptou]

• added a new feature serde to the libp2p crate
• when this feature is enabled, Serialize and Deserialize are implemented for PeerId and gossipsub::MessageId
• fixes an issue with GossipsubConfigBuilder::build() where it requires &self to be borrowed for 'static lifetime due to lifetime elision

[thomaseizinger]

Thanks!

One comment on the feature name in libp2p-core!

[thomaseizinger]

Some more comments :)

[thomaseizinger]

Suggested change

	#[cfg(feature = "serde")]
	use serde::{Serialize, Deserialize};
	/// Public keys with byte-lengths smaller than `MAX_INLINE_KEY_LENGTH` will be
	/// automatically used as the peer id using an identity multihash.
	const MAX_INLINE_KEY_LENGTH: usize = 42;
	/// Identifier of a peer of the network.
	///
	/// The data is a multihash of the public key of the peer.
	#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
	/// Public keys with byte-lengths smaller than `MAX_INLINE_KEY_LENGTH` will be
	/// automatically used as the peer id using an identity multihash.
	const MAX_INLINE_KEY_LENGTH: usize = 42;
	/// Identifier of a peer of the network.
	///
	/// The data is a multihash of the public key of the peer.
	#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]

This would contain it to a single line.

[mxinden]

//CC @elenaf9 as you proposed this in #2341.

[mxinden]

fixes an issue with GossipsubConfigBuilder::build() where it requires &self to be borrowed for 'static lifetime due to lifetime elision

@laptou would you mind proposing this change in a separate pull request?

[laptou]

I've created a new branch on my fork that has just the changes relevant to adding Serde, but I can't change the branch associated with this PR, so I will close it and open another one.