trust-dns-resolver is a fairly heavy crate and is only used for resolving `mongodb+srv` connection strings; where users know that won't be needed they could drop the dependency if it were optional.

Unfortunately, moving existing functionality behind an optional feature is a breaking change, so this need to wait for 3.0. When we do that, we should also use the technique from https://slint.dev/blog/rust-adding-default-cargo-feature to mean that we can do similar changes in the future without them being breaking.