core: Remove `Transport::{Dial,ListenUpgrade}` in favor of using trait objects

Context

Whilst toying around with https://github.com/libp2p/rust-libp2p/issues/2650 which was motivated by trying to solve the merge conflicts in https://github.com/libp2p/rust-libp2p/pull/3254, I came across our EitherFuture type. This one exists because the trait bounds on Transport::Dial and Transport::ListenUpgrade force the designated type to "transpose" the output. In other words, with Either implementing Transport, the output of Dial is required to be Result<Either<A, B>, ...> instead of Either<Result<A, ..>, Result<B, ..>>. This makes sense but is not compatible with a general implementation of Future on a type like Either. Thus, we need to provide our own, specialized type for this.

Proposal

Should we remove the associated types Dial and ListenUpgrade in favor of always using BoxFuture?

Benefits

• Less code to maintain
• Simplified interface
• Smaller public API of Transport implementations

Drawbacks

• Forces all Transports to be Send
• More allocations as part of creating new network connections

Start by reading the Transport trait, its Dial and ListenUpgrade associated types, and the EitherFuture type described in the issue. Trace the affected Transport implementations and determine whether replacing those associated types with BoxFuture preserves the stated behavior and addresses the listed trade-offs; done means the proposal is implemented consistently across the public API.