Re-export Either type for better DX

[bajtos]

Description

In #3337, EitherError was replaced with either::Either. This is fine, except that libp2p does not re-export the Either type, and therefore libp2p consumers have to add either to their dependencies. It's a bit of maintenance overhead and a risk of a versioning conflict.

I propose re-exporting the entire either crate from libp2p.

This will also simplify the upgrade path from v0.50 to v0.51.

- use libp2p::core::either::EitherError;
+ use libp2p::core::either::Either;

Are you planning to do it yourself in a pull request?

Yes, I am happy to contribute a pull request if we agree this is a change you will accept.

Should we export either as libp2::core::either or libp2p::either?

[thomaseizinger]

Given that either is already > 1.0 I am tempted to say that depending on it yourself is the way to go.

[drHuangMHT]

Consider closing this issue?