dns: add `web-sys` support to `dns` Transport

### Description

Right now, libp2p-dns supports `tokio` and `async-std` but we won't be able to resolve from a `wasm` executor in `web-sys`.

### Motivation

I want to resolve `dnsaddr` multiaddrs from `wasm`

### Current Implementation

Currently only `tokio` and `async-std`

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

Maybe