RUST-1680: put trust-dns-resolver under feature flag

[Logarithmus]

As far as I understand, this is only used for mongodb+srv:// connection strings. I don't use it, and trust-dns-resolver takes significant time to compile, and is whopping 34k lines of code! https://lib.rs/crates/trust-dns-resolver.
So making it an optional dependency would be very appreciated. I can make a PR if you say my reasoning is correct.

[abr-egn]

Hi!

You're right that moving trust-dns-resolver to optional would make sense; unfortunately, that would be a breaking change, so we need to defer that to when we make a 3.0 release (won't be in the near future). I've filed https://jira.mongodb.org/browse/RUST-1680 to track it so we include it when we do get to that point!

[Logarithmus]

@abr-egn in fact we can make trust-dns-resolver feature enabled by default, this way it won't be a breaking change and everyone will be happy!

[abr-egn]

Even with that, it'll still be breaking for users who have disabled default features.